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Automatic Habit of Meditation
In psychology, a habit is defined as a behaviour done automatically, requiring little conscious effort. I have meditated recently pretty much every second day, but I fail to see any automaticity; its incredibly effortful to sit and meditate for me still.
Does Buddhism have anything to say about forming such habits? Is there any knowledge -- whether in the scriptures or from personal experience -- suggesting the ideal frequency of meditation (to form a habit)?
Here, I mean the frequency which would enable consistent and automatic sitting and meditation.
Also, does consistency relative to the hour of the day of meditation matter at all?
Any information is welcome, thank you.

IMO, yes, it does. :) Many meditation programs and retreats are organized around this principle. It takes the average person somewhere between 20 and 30 days to form a solid habit. The more often and more regular the repetition, the more solid the habit. It is the same with sports, gym, eating, etc. The more you practice meditation, the more of an automatic habit it will become.
Just like with working out, set a specific time every day to meditate. Set a progressively longer series of sit times, so you start out light and work your way up to an hour at a time. You can also try maintaining mindfulness throughout the day, retreading the neural pathways as often as you can, even if just one minute each hour. Once mindfulness and samadhi become habitual, you are on your way.
And remember, it's only heavy if you pick it up. It's only hard if you don't want to be there. Observe your mind when you feel it hard to sit. Why is that? Are you craving snacks, sleep, or entertainment? Exhausted by pain or striving? Remember at those times why you are meditating, and remember to relax and take it easy. Make it fun and rewarding. Make it a game, how many breaths can you observe without mental interruption? Set a high score board of Stillness if it helps. ;)

Does Buddhism have anything to say about forming such habits? Is there any knowledge -- whether in the scriptures or from personal experience -- suggesting the ideal frequency of meditation (to form a habit)?

I realize you are asking about forming habits, but sometimes the road to effortless habits goes through eliminating hindrances. If we're talking satipatthana, a big deal is actually investigating hindrances to overcome these obstacles, as it is also conducive towards further cultivation of your meditation regime.
Generally speaking (vitarka), the five hindrances are:

Sensual Desire
Ill-will
Sloth and Torpor
Restlessness and Remorse
Doubt

Insights regarding hindrances are most likely not only beneficial for your meditation practice, but can likely be generalized to life as a whole.
More reading, and not least suggested antidotes to the hindrances:
https://www.accesstoinsight.org/lib/authors/nyanaponika/wheel026.html

Also, does consistency relative to the hour of the day of meditation matter at all?

You may benefit from looking at prolonging the time for meditation rather than increase the frequency or timing during the day. It will expose you to important observations for deepening dhyana.
I'd argue that the more overall time spent in dhyana the better.

Start with one second.  Do it every day for a week.  Move up to 15 seconds.  Do it everyday for a week.  Keep adding 15 seconds until you hit two minutes, then start adding thirty seconds.  Keep adding thirty seconds until you hit a half an hour.  What you do after you hit 30 minutes is up to you.  There are only two things you need to do - sit everyday and don't exceed the amount of time that you are supposed to sit.  If a sit goes well, end it, and let those positive feelings kindle your desire to sit again the following day.
The only other recommendation I would make would be to find a group to sit with..  Having people to whom you are accountable makes it much easier to maintain a practice.  If you do join a group, it's perfectly OK to sit more than your allocated time for the day.  There is no magic formula to establish a habit.  Right effort is karmimcally recursive - like a spiritual, centripetal engine.  The initial excursion is hard, but once established, the amount of effort required to keep things going is greatly reduced.
But you first have to try.

Firstly, you do not specify in your question how long you have been doing "every other day" meditation or how long your sessions are. Meditating for 5 min ever other day will NEVER develop "automaticity" nor can one expect that meditating for 90 min every other day will develop automaticity in a matter of weeks(if ever). Also of note- the momentum generated by SUSTAINED "informal" mindfulness/meditation during daily activities plays a big role in developing automaticity. In my experience, some genuine pervasive automaticity can be experienced by a couple months of daily sits of approx 1 hr, followed by a retreat of a week or so which has you primarily in sitting and walking meditation for 5+ hrs per day.


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5th precept missing from 8fold Path?
I can find the first 4 precepts explicitly listed in the 8fold Path- but not the 5th precept regarding intoxicants. Am I missing it in there somewhere? I am wondering if it was a later addition to the tradition.

This question might have been asked before, here: Drunk monks and the 5th precept

There is an interesting relationship between the first four precepts and the fifth precept.
For e.g. from the Kalama Sutta (quoted below), we can see that the fifth precept is not included. You can find a similar grouping of the first four precepts in the Sigalovada Sutta as the four vices. It also appears in this way in the Sankha Sutta.
But there's a reason for this. Consuming intoxicating substances is by itself not a vice or evil act (unlike killing, stealing, telling lies and sexual misconduct), but it can cause you to become heedless and violate the first four precepts.
In this sense, the fifth precept is only there to support the first four precepts (and the rest of the training).
From the Kalama Sutta:

“A greedy individual, overcome by greed, kills living creatures,
  steals, commits adultery, lies, and encourages others to do the same.
  Is that for their lasting harm and suffering?” “Yes, sir.”
“A hateful individual, overcome by hate, kills living creatures,
  steals, commits adultery, lies, and encourages others to do the same.
  Is that for their lasting harm and suffering?” “Yes, sir.”
“A deluded individual, overcome by delusion, kills living creatures,
  steals, commits adultery, lies, and encourages others to do the same.
  Is that for their lasting harm and suffering?” “Yes, sir.”

From the Sikkha-Dubbalya Sutta:

"Monks, these five are things that weaken the training. Which five?
  The taking of life, stealing, sexual misconduct, the telling of lies,
  and distilled & fermented beverages that are a cause for heedlessness.
  These five are things that weaken the training.

Please see this question for more info on heedfulness (appamāda).

Its also excluded from the ten skilful actions. https://suttacentral.net/an10.178/en/sujato

AN4.201 lists these five:

And what is a better person?
It’s someone who doesn’t kill living creatures, steal, commit sexual misconduct, lie, or use alcoholic drinks that cause negligence.
And they encourage others to avoid these things.
This is called a better person.

This works for me. Does it work for you?

The Noble Eightfold path actually has 7 precepts (3 bodily + 4 verbal). In many suttas, the Buddha talk about the 7 precepts instead of the 5.
The 7 precepts are earlier because they are included in the Noble Eightfold Path. As the monastic sangha grew, new precepts were gradually added (including the 5th precept); most of the precepts were added after some issues had occurred and to prevent the same incidents from happening again.
Origin of 5th precept as I heard from Ven Dhammavuddho (abbot of Vihara Buddha Gotama): There was a monk who possessed psychic powers (means he attained at least the 4th Jhana). He had just tamed a Naga (snake spirit) and the villagers were very grateful. They wanted to make an offering and asked him what he wanted. He didn’t want anything but his fellow monks suggested he asked for alcoholic beverage. So, the monks drank the alcohol. His fellow monks were seasoned drinker; so they were not drunk. However, this monk became drunk. When they went back to see the Buddha, this monk had his feet pointing towards the Buddha (which he would not have done if he was sober). The Buddha asked the other monks whether this drunken monk could fight the Naga now. The answer is obvious; he couldn’t. From this incident, the Buddha added the precept to abstain from consuming intoxicants, which leads to heedlessness.

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What does "self" mean?
I have had the impression that "self" has at least sometimes been thought of as "something" other than the 5 aggregates.
What does "self" mean?
What is "sense of self"?

The Dhamma says that dhammas are non-self -- and that questions-about-self is unwise attention which leads to a thicket of views -- so I don't know how to answer this question!

But then how is one supposed to understand the words "dhammas are non-self" then? How is one supposed to understand any statement that uses the word "self" without knowing what is meant by "self"?

The fear about questions about self is misplaced. What is intended by that admonition is refraining from speculations about the origin or destiny, or existence or non-existence of a self.
What the questioner is asking, I believe, is the definition in Buddhism of Self, and that is a legitimate question to ask and answer.
The answer is that it is that which one believes is under one's control (the scope goes beyond the individual body; usually expressed as 'self or what relates to self'). And further, though it is description by negative, it is the idea that a self under one's control would not suffer pain or come to an end.
The Dhamma points out that there is only one thing that is under one's control, and that is one's acts of mind, speech or body. The outcome of one's acts, however, is out of one's control.

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How did Sarakani become a stream enterer despite having an alcohol drinking habit?
I read SN 55.24 (quoted below) and it says Sarakani became a stream enterer despite having an alcohol drinking habit, because he undertook the training before he died.
What does the sutta really mean? Does it mean he gave up alcohol and undertook the training, then became a stream enterer?
The sutta isn't very clear on whether Sarakani gave up alcohol. Did he give it up? Or not?

Now at that time Sarakāni the Sakyan had passed away. The Buddha
  declared that he was a stream-enterer, not liable to be reborn in the
  underworld, bound for awakening.
At that, several Sakyans came together complaining, grumbling, and
  objecting, “It’s incredible, it’s amazing! Who can’t become a
  stream-enterer these days? For the Buddha even declared Sarakāni to be
  a stream-enterer after he passed away. Sarakāni was too weak for the
  training; he used to drink alcohol.”

Your questions appear unanswerable. However, it appears probable he gave up drinking prior to stream-entry.

Does it meant that he gave up alcohol and undertook the training, and became a stream enterer?
That's not how I read the translation, i.e. I'm not sure that is what's implied.
Perhaps I should start by mentioning that, whereas SN 55.24 says ...

Sarakāni was too weak for the training; he used to drink alcohol.”
  Saraṇāni sakko sikkhādubbalyamāpādi, majjapānaṃ apāyī”ti.

... there's SN 55.25 which says ...

Sarakāni didn’t fulfill the training.”
  Saraṇāni sakko sikkhāya aparipūrakārī ahosī”ti.

Anyway, near the start of SN 55.24 it says,

Mahānāma, when a lay follower has for a long time gone for refuge to the Buddha, the teaching, and the Saṅgha, how could they go to the underworld? And if anyone should rightly be said to have for a long time gone for refuge to the Buddha, the teaching, and the Saṅgha, it’s Sarakāni the Sakyan

So that's one thing and maybe enough.
It isn't the same story but see also the Weaver's Daughter ...

At the conclusion of the discourse that maiden was established in the fruit of stream-entry

... and there a factor (and something that may have in common with this) was her having lived with the Buddha's message for a long time.
The next paragraphs talks about immersion and so on -- variations on a theme (all including immersion as a kind of minimum). The same person (Mahānāma the Sakyan) asked a related question in AN 3.73.
It ends with -- or perhaps, "But it ends with" (though it might be better to understand it without "but", because "without but" is a description of the end-state i.e. having completed stream-entry):

If these great sal trees could understand what was well said and poorly said, I’d declare them to be stream-enterers. Why can’t this apply to Sarakāni?

I get the impression that stream-entry depends on understanding the Dhamma when it's spoken by the Buddha -- there are many examples of that (at the end of SN 56.11 for a start, but also often at the end of many suttas with the Bhagava and a lay-person).
So that's another thing (maybe the same thing), and also enough.
Finally it says,

Mahānāma, Sarakāni the Sakyan undertook the training at the time of his death.
Saraṇāni, mahānāma, sakko maraṇakāle sikkhaṃ samādiyī”ti.

The translation says "training" but the Pali says saraṇa -- maybe "refuge".
I think that's saying he became a stream enterer when he died, before he died, as he was dying -- however long that took, whether it was a moment before (Last thought before death?), or whether the "time of dying" (maraṇakāla) was days or weeks or longer, months.
I think we're meant to believe it's possible for a person -- for almost anyone (see also Aṅgulimāla, "The Buddha had often warned not to judge people from appearances and their external behavior").
And the suttas have many examples of people attaining a higher "stage" of enlightenment as they're dying.
My opinion, if you'll forgive it, is that I may be disinclined to recommend a life of drunkenness -- that might be crazy, and perhaps not in a good way. That said, it's possible (and fortunate) that stream entry may occur towards the end of life (though arguably not a safe bet and compassionate and everything to wait until then).
The sutta isn't very clear on whether Sarakani gave up alcohol. Did he give it up?
Perhaps you give up a lot as you're dying.

People, other than the wise, do not realize, "We in this world must all die," (and, not realizing it, continue their quarrels). The wise realize it and thereby their quarrels cease.

See also e.g. SN 25.3, "He is incapable of passing away until he has realized the fruit of stream-entry." (or translated, "They can’t die without realizing" -- I think the Pali says literally, something like, "Unable lifetime do not fruit-of-steam-entry he realises", so don't read too much into the wording of the translation).

I think the natural assumption is that Sarakaani became either a Faith-Follower and or a Dhamma-Follower during his life and thus realized fruition of stream-entry as he was dying.

"One who has conviction & belief that these phenomena are this way is called a faith-follower: one who has entered the orderliness of rightness, entered the plane of people of integrity, transcended the plane of the run-of-the-mill. He is incapable of doing any deed by which he might be reborn in hell, in the animal womb, or in the realm of hungry shades. He is incapable of passing away until he has realized the fruit of stream-entry.

"One who, after pondering with a modicum of discernment, has accepted that these phenomena are this way is called a Dhamma-follower: one who has entered the orderliness of rightness, entered the plane of people of integrity, transcended the plane of the run-of-the-mill. He is incapable of doing any deed by which he might be reborn in hell, in the animal womb, or in the realm of hungry shades. He is incapable of passing away until he has realized the fruit of stream-entry.

"One who knows and sees that these phenomena are this way is called a stream-enterer, steadfast, never again destined for states of woe, headed for self-awakening."SN25.1

As i see it, it is the natural assumption because;

I think it's clear that these are the exact type of people who having to an extent neglected the training would be realizing fruition as they die.

This interpretation is also inferrable from the Sarakaani Sutta because it is said in those very Sutta;

"Take the case of another man. He is not even endowed with unwavering devotion to the Buddha, the Dhamma, the Sangha. He is not joyous and swift in wisdom and has not gained release. But perhaps he has these things: the faculty of faith, of energy, of mindfulness, of concentration, of wisdom. And the things proclaimed by the Tathaagata are moderately approved by him with insight. That man does not go to the realm of hungry ghosts, to the downfall, to the evil way, to states of woe.

"Take the case of another man. He is not even endowed with unwavering devotion to the Buddha, the Dhamma, the Sangha. He is not joyous and swift in wisdom and has not gained release. But he has just these things: the faculty of faith, of energy, of mindfulness, of concentration, of wisdom. Yet if he has merely faith, merely affection for the Tathaagata, that man, too, does not go to... states of woe.SN55.24

Which is in so many words a clear descriction of the pair. Furthermore Sarakaani is further described as a good seed;

a teaching that’s well explained and well propounded, emancipating, leading to peace, proclaimed by someone who is a fully awakened Buddha. This is what I call a fertile field. A disciple remains in such a teaching, practicing in line with that teaching, practicing it properly, living in line with that teaching. This is what I call a good seed. Why can’t this apply to Sarakāni? SN55.25

Therefore i say it is natural to assume that Sarakaani was faith and or a dhamma-follower as he took to drink.
This interpretation is disagreeable to those who hold that a Sotapanna cannot drink alcohol because the pair [faith- and the dhamma-follower] are also considered Ariya and those who hold the position that Sotapanna cannot drink also, from what i've seen, hold that Ariya cannot drink at all. That also because it seems like some discourses might not differentiate between a stream-enterer and the pair.
Therefore people will hold that Sarakaani was a putthujjana [worldling] and a common alcoholic at that. That he was without any substantiaĺ belief nor understanding of the Dhamma up until the point of death and then realized fruition of stream-entry.
As to whether a Sotapanna can drink alcohol these are the considerations;

It has never been stated that they can not or that they wouldn't want to. I think it's reasonable to assume that they at the very least can be forced to but some hold that it is impossible and it cannot happen.

Of the Sotapanna it is proclaimed that they are incapable of the six Abhithanani; i. matricide, ii. patricide, iii. the murder of arahants, iv. the shedding of the Buddha's blood, v. causing schism in the Sangha, and vi. pernicious false beliefs (niyata micca ditthi) and furthermore;

Any evil action he may still do by deed, word or thought, he is incapable of concealing it; since it has been proclaimed that such concealing is impossible for one who has seen the Path (of Nibbana). Snp2.1

Here it's noteworthy that it is explicitly expressed "since it has been proclaimed that such concealing is impossible".
Also this is proclaimed;

"There is the case where a monk is wholly accomplished in virtue, moderately accomplished in concentration, and moderately accomplished in discernment. With reference to the lesser and minor training rules, he falls into offenses and rehabilitates himself. Why is that? Because I have not declared that to be a disqualification in these circumstances. But as for the training rules that are basic to the holy life and proper to the holy life, he is one of permanent virtue, one of steadfast virtue. Having undertaken them, he trains in reference to the training rules.
"With the wasting away of [the first] three fetters, he is one who has seven more times at most. Having transmigrated and wandered on among devas and human beings, he will put an end to stress. AN3.86

Noteworthy here is that the pali is;

They break some lesser and minor training rules, but are restored.
So yāni tāni khuddānukhuddakāni sikkhāpadāni tāni āpajjatipi vuṭṭhātipi

The wording is the same as DN16;

If it wishes, after my passing the Saṅgha may abolish the lesser and minor training rules.
Ākaṅkhamāno, ānanda, saṃgho mamaccayena khuddānukhuddakāni sikkhāpadāni samūhanatu.

This is very important because we know that the First Buddhist Council, held shortly after Buddha's parinibbana, couldn't agree on what these minor and lesser rules were(!).
Second point to note is that here in the AN3.86,  it is proclaimed that the transgressions are possible 'since it has not been declared to be a disqualification' i would propose that the principle of not being proclaimed is emphasised as it was in 'since it has been proclaimed that such concealing is impossible' in the Snp2.1 and is of particular importance.
As for the meaning of disqualification i see two interpretations i. Expulsion from the Order, ii. Disqualification from the Status.

Intoxication isn't listed among vices that wise never praise;

"What are the four vices in conduct that he has eradicated? The destruction of life, householder, is a vice and so are stealing, sexual misconduct, and lying. These are the four vices that he has eradicated."
Thus spoke the Exalted One. And when the Master had thus spoken, he spoke yet again:
Killing, stealing, lying and adultery,
These four evils that the wise never praise. DN31

I think it's clear based on this evidence that it cannot be assumed that a Sotapanna cannot drink and one who holds that position needs to prove it as it is an extraordinary assumption because it has not been proclaimed to be impossible.
Here is more circumstantial evidence;

The nāga (living in the fire building) saw that Ven. Sāgata had entered and, on seeing him, was upset, disgruntled, and emitted smoke. Ven. Sāgata emitted smoke. The nāga, unable to bear his rage, blazed up. Ven. Sāgata, entering the fire element, blazed up. Then Ven. Sāgata, having consumed the nāga’s fire with his own fire, left for Bhaddavatikā.[...]

The lay followers of Kosambī heard, ‘They say that Ven. Sāgata did battle with the Ambatittha nāga!’,[...]
One, went to Ven. Sāgata and, on arrival, having bowed down to him, sat to one side. As they were sitting there they said to him, ‘What, venerable sir, is something the masters like that is hard for you to get? What can we prepare for you?‘

“When this was said, some group-of-six bhikkhus said to the Kosambī lay followers, ‘Friends, there is a strong liquor called pigeon’s liquor (the color of pigeons’ feet, according to the Commentary) that the bhikkhus like and is hard for them to get. Prepare that.’

“Then the Kosambī lay followers, having prepared pigeon’s liquor in house after house, and seeing that Ven. Sāgata had gone out for alms, said to him, ‘Master Sāgata, drink some pigeon’s liquor! Master Sāgata, drink some pigeon’s liquor’ Then Ven. Sāgata, having drunk pigeon’s liquor in house after house, passed out at the city gate as he was leaving the city.
Vinaya, prohibition precept origin

It also has to be assumed that there were more attainments before the rule was established because it is said;

What is the cause, lord, what is the reason, why before there were fewer training rules and yet more monks established in final gnosis, whereas now there are more training rules and yet fewer monks established in final gnosis?[...]
When beings are degenerating and the true Dhamma is disappearing, there are more training rules and yet fewer monks established in final gnosis. SN16.13

In the Snp2.1 it is also said that they can be "excessively heedless", alcohol is said explicitly to cause heedlessness. Just pointing out that maybe if one causes the other...

Those who hold that Ariya cannot drink do it based on commentary and there are different commentary positions on this matter where some do not exclude the possibility.
This is a very taboo subject because many contemporary teachers hold the view that it is impossible and cannot happen. The list would include many prominent teachers of The Thai Forest and Mahasi Sayadaw. Some contemporary teachers go as far as entertaining the idea that alcohol would simply evaporate from the lips of the Ariyans if forced to drink.
The implications of Sotapanna being unable to drink are very interesting to analyze because as i see it, it allows for equating ariyakhantisila [virtues dear to noble one's] to the five precepts and this has quite big social & moral implication in the Sangha.
Remember that 1st Council didn't establish which rules could be abandoned, now think a little about how many and which rules become minor & lesser if the fundamental rules are defined as the 5 precepts (which is from what i've seen the interpretation by those who hold that a Sotapanna cannot drink as they typically tend to equate ariyakantisila to pancasilani ['virtues dear to noble one's' with which all ariyans are endowed and 'five precepts']).
I think it is clear who has the burden of proof and what the most natural interpretation is based on the tipitaka.
Fwiw and not that it matters but to exhaust what i know of this;

I think the commentary holds that Sarakaani died with alcohol in his system but this is just something i heard being taught.
I also vaguely recall hearing a commentary story of a female lay disciple, at some point proclaimed to be Sotapanna being at some point implicated in a story where some drinking did or might have happened. Iirc this would be Nandiya and it might be in relation to the discourse given to her where it is said that "the heedless noble disciple dwells in pain". It could be someone else tho, i recall she went with her friends to like picnic of sorts.
in the Dhp commentary to "the hand without a wound and the carrying of poison" verse  it is said that he says that 'Sotapannas do not kill, they do not want others to be killed'.

The prerequisite of stream entry is Right View, the first factor of the Noble Eightfold Path. To have Right View, one needs to be able to understand the Dhamma, which is why the Buddha said “even if these great sal trees could understand what is well spoken ... I would declare these trees to be stream enterers”.
The Sila component of the Noble Eightfold Path is Right Speech, Right Action, Right Livelihood; they do not include the 5th precept. In fact, they contain 7 precepts (3 bodily, same as 1st, 2nd, 3rd precept, and 4 verbal).
In many suttas, the Buddha talk about these 7 precepts instead of the 5. The 7 precepts are earlier because they are in the Noble Eightfold Path. As the monastic sangha grew, many precepts (including the 5th precept) were gradually added.
Anyway, the main message in this Sutta is that listening and understanding the Dhamma to get Right View is the prerequisite to stream entry.

The virtue of Ariya is not measured only by the restraint of bad behavior, one's good qualities must also be recognized.
Even a sotapanna can be excessively heedless but they can also do great skillful things that the unistructed person can't do.
Not drinking alcohol is relatively easy and even ordinary people can do this.
Seeing with discernment the four noble truths, that is difficult and these difficult things make one dear to wise people.
Sarakaani was a good Buddhist, followed on faith and developed modicum understanding as dhamma-follower.
However being fiercely heedless it was not until the time of dying that he undertook the training to attain knowledge & vision due to the urgency & accute perception of death i presume.

Have you ever train to stop some habit for long time without successful and everyone remember you as a looser of that habit, but you still try and try again and again?
And have you ever hit a success moment after that long time effort, can you specify the specific time of the success moment of your long time training before it's ending?
When you try to stop drinking for long long time, you have been fail but you never give up, people will still remember you as an alcoholic on the day you successful stopping drinking.
It needs time  to chang the rumors even after a moment of enlightenment. You still be a child in your mother eyes even you are 30 years old.
And if there are past lives, you can train for uncountable lives and finish it in the last life immediately easily.


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What is laughing wisdom and swift wisdom?
From SN 55.24, I found the terms "laughing wisdom" and "swift wisdom". What do they mean?

They have laughing wisdom and swift wisdom, and are endowed with
  freedom.
hāsapañño javanapañño vimuttiyā ca samannāgato.

I'm guessing this means they see the humour and irony of our situation and are realised to the point they may act and speak (or not act and speak) spontaneously and reliably from truth.

Footnotes 50 and 51 from Piya Tan's analysis of SN 55.24:

Of joyous wisdom,” hāsa,pañña, alt tr “of laughing wisdom” (from hasa, “laughter, mirth, joy”). “Here one
  with much joy [laughter], mirth, contentment and gladness perfects the virtues: this is joyous wisdom” (Pm 21.17/
  2:199 f). Comys mention hāsa,paā (joyous wisdom), javana,paā (quick wisdom) [see foll n], tikkha,paā
  (sharp wisdom, which cuts off all defilements) and nibbedhika,paā (penetrating wisdom, by which one maintains
  dispassion toward all formations) (DA 3:391-393, 4:84-86; MA 4:84; SA 1:120-123, 2:85 f; AA 2:85; ThaA 3:106 f;
  NmA 1:231 f; cf PmA 3:640 on hasa,paā)
“Of quick wisdom,” javana,pañña. This refers to one who quickly understands that the 5 aggregates are all impermanent, suffering and non-self (Pm 21.18/2:200). See prev n.

"Pm" is the Patisambhidā Magga.


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How do monks transcend the desire for relationships & sex?
Being a 'genuine' monk means a certain kind of dispassion towards the world.
How is sexuality & 'relationship craving' effectively dealt within a monk?
I am pretty sure a lot of monks struggle with it although they never speak of it. That's probably why many monks disrobe because they can't get the fruits of the noble path.
Is it with metta towards the opposite sex or just seeing the unattractiveness of it that craving reduces?

Is it with metta towards the opposite sex or just seeing the unattractiveness of it that craving reduces?
I think it's both of those -- with "unattractive" meaning:

Something like graveyard meditation
Seeing it as dissatisfactory (dukkha) or "not permanently satisfying"
Seeing consequences

I get the impression from watching some of Ven. Yuttadhammo's videos that it's also a matter of developing some dispassion towards "feeling".
And I think perhaps it's a matter of discipline (e.g. the vinaya) as an alternative to pursuing sensuality.
Another example of that, for a monk, is feeling hungry and yet not eating (except properly).
I suppose it's also a matter of habit, changing or attenuating existing habits (if any), and avoiding causes or occasions (e.g. being alone with a woman).

How is sexuality & 'relationship craving' effectively dealt within a monk?
It's easier to start dealing with this while still a lay person. Robes are not a guard against sensual delights. The Vinaya provides strong guards observed by avoiding sensual situations, but practicing restraint and relinquishing before going forth are quite helpful.
The drawback of Western society is that we have coined the term "sexual needs" by elevating sexual desire to the level of food. As Sariputta said in DN33:

All sentient beings are sustained by food.

Sexual desire is not mentioned in the above. Just food. Therefore "sexual needs" is a delusional mask for sexual desire. Sexual desire is fire, and like all fire is best restrained while small.  It's easier to put out a match than a burning house. Indulging in sexual desire is like quenching a fire by letting it burn the house down. This is why understanding dependent origination can really help deal with sexual desire. The earlier we understand that we are hooked by desire, the earlier and easier the escape.
To understand where desire starts, we have to understand how it grows:

As long as consciousness remains, it remains involved with form, supported by form, founded on form. And with a sprinkle of relishing, it grows, increases, and matures.

Notice the part that form plays in consciousness. Sexual desire manifests as a craving for particular forms (e.g., we may desire people having a certain shape or sound or smell or feeling). In other words, we indulge in a prejudice of favoritism and aversion in our relationships, prizing desirable people over undesirable people.
The drawback of that desire is prejudice. Prejudice is divisive. The escape from that desire is non-prejudice. By dismantling and relinquishing our prejudices, we can escape the torment of sexual desire. We can make friends with those we might otherwise ignore. We can avoid unethical yet attractive people. We can see that attraction diminishes with age and infirmity. We can even go to the rather intense contemplation of impermanence and decay:

It’s when a mendicant preserves a meditation subject that’s a fine foundation of immersion: the perception of a skeleton, a worm-infested corpse, a livid corpse, a split open corpse, or a bloated corpse.

However, since we live in a world that fears and shuns the contemplation of bloated corpses, it may be easier to work on prejudices. Find the beauty in those you would otherwise avoid. After a time, you might see beauty everywhere and experience a desire for nothing in particular.

They're focused only on beauty.
  This is the third liberation.

Regarding relationships as needs, notice that relationships are defined by giving. And the highest form of giving is:

But they give a gift thinking, ‘This is an adornment and requisite for the mind.’

This includes the gift of alms food. Food sustains sentient beings. Yet monks receiving alms food must also know that food to be an adornment, impermanent and uncertain. Understanding the impermanence and uncertainty of the gift of food provides a daily and powerful encouragement to reduce relationship craving. A monk is necessarily grateful to all who provide alms food--that too erases prejudice and craving. And that gratitude also encourages monks to offer the gift of Dhamma to all in return for the gift of alms food. This is the best relationship.
And yes. That was all in DN33

You mention "sexual needs" aren't a real need, There may be some social need -- "need a friend", intimacy, belonging -- perhaps the sangha provides some of that, possibly like an adult family might, although different -- the OP's title above is "desire for relationships" and not only "desire for sex". Anyway perhaps that's part of how monks can manage i.e. that there are relationships, some "need for relationship" is fulfilled, there's even a more-or-less friendly relationship with lay society, would you agree?

But they successfully distinguish between "sex" and "relationship" where some lay people -- or lay common language, and a caricature of lay expectations -- maybe doesn't make that distinction.

Thanks ChrisW. Focusing on sexual needs, I had neglected relationships. Hopefully my latest edit addresses this lack.

It does! And it's hard for me to see how what it says about gratitude might be wrong -- except gratitude might imply a dependence and attachment which monks are not supposed to model, see e.g. this answer, I think you [a layperson] can't expect them [a monk] to look you in the eye and smile and nod and say "thank you", also the comment after that answer. Perhaps there are relationships between monks within the sangha (preceptors and so on, also e.g. medicine and friendship).

Indeed. As a layperson I have come to realize that the question posed by the OP must be addressed starting with laity. Going forth is not a solution for the OP in itself, and laity will have to deal with all the issues outlined in the posted response. To answer the OP completely and well would require all the EBTs since it is a subtle and quite important question that touches on many issues.

I think the OP might be a layperson seeking an answer relevant to laity. A monk responding to a monk would have a different answer such as "practice jhana". And the monk's answer would not be helpful to laity struggling with these issues.


text:
In which sutta does a lay disciple of the Buddha tell a non-buddhist mendicant that he has experienced jhana?
I remember reading a sutta some time ago in which a lay disciple of the Buddha held a conversation with a non-Buddhist ascetic. I don't remember the topic of their conversation, but at one point the lay buddhist talked about having experienced jhana. Does anyone know which sutta this is?
Thanks in advance for your answers!

your best first try is this essay
perhaps this
When Do the Jhānas Become Necessary?

While there seem to be no suttas that impose an inflexible rule to the
  effect that a lay noble disciple must possess the jhānas, there are at
  least two texts that explicitly ascribe all four jhānas to certain
  householders. One, found in the Citta-saṃyutta (SN 41:9/IV 300-2),
  features Citta the householder, the foremost lay preacher, in a
  conversation with a naked ascetic named Kassapa. Kassapa was an old
  friend of Citta who had embraced the life of renunciation thirty years
  earlier, and this is apparently their first meeting since that time.
  Kassapa confesses to Citta that in all these years he has not achieved
  any "superhuman distinction in knowledge and vision befitting the
  noble ones" (uttarimanussadhammā alamariya-ñāṇadassanavisesa); all he
  does is go about naked, with a shaved head, using a feather brush to
  sweep his seat. He then asks Citta whether, as a lay disciple of the
  Buddha, he has reached any distinguished attainments. Citta says that
  he has, and then declares his ability to enter and dwell in the four
  jhānas (he uses the standard formula). To this he adds: "Further, if I
  were to die before the Blessed One, it would not be surprising if the
  Blessed One would declare of me: 'There is no fetter bound by which
  Citta the householder might come back to this world.'"[32] Through
  this bit of coded text, partly a stock formulation, Citta is informing
  his friend that he is a non-returner with access to the four jhānas.
The other sutta is AN 7:50/IV 66-67 and concerns the lay woman
  Nandamātā. In the presence of the Venerable Sāriputta and other monks,
  Nandamātā has been disclosing the seven wonderful and marvellous
  qualities with which she is endowed. The sixth of these is possession
  of the four jhānas, again described by the stock formula. The seventh
  is as follows: "As to the five lower fetters taught by the the Blessed
  One, I do not see among them any as yet unabandoned in myself."[33]
  This too is a coded way of declaring her status as a non-returner.

https://www.budsas.org/ebud/ebdha267.htm

Thank you very much, SN 41.9 was indeed the sutta I was thinking about.


text:
Please explain joy and/or the Kimatthiyasutta
Can you explain something about joy?
For example it's mentioned in AN 11.1:

“But what’s the purpose and benefit of having no regrets?”
  “Avippaṭisāro pana, bhante, kimatthiyo kimānisaṃso”?
“Joy is the purpose and benefit of having no regrets.”
  “Avippaṭisāro kho, ānanda, pāmojjattho pāmojjānisaṃso”.

I can imagine several specific question, please answer any or none of these (from scripture or experience, any tradition of Buddhism).
I'm especially interested in the context of everyday lay life, rather than specifically/only during formal meditation.

Can you explain the joy/rapture/tranquillity/bliss/immersion sequence? What's the difference between these, what's the connection? Is some of this specifically meditative? I'm guessing that at least the start of the sequence, "skilful ethics" isn't only meditative. If this is too long to explain in an answer is there a text/reference you recommend instead?
Does some condition or thing cause joy, apart from a lack of remorse? Is it a matter of choice, at all? When dukkha arises would you say "that's dukkha, I'd better wait for it to cease", or would you say, "that's dukkha, I'd better 'choose joy' instead?"
Is it "primarily ethical", if that makes sense as a question, e.g. if ethics is doing the right thing then is joy (or choosing joy, or intending joy) also "the right thing" in that way?
Would I be right to assume that these maybe happen in stages, like one before the other -- with earlier ones having each next one as their eventual purpose? Is it something of a gradual training, does it make sense to focus on sometimes improving or cultivating one stage (e.g. joy)? How do you "cultivate" joy (is it only by cultivating skilful ethics and non-remorse or...)?
I'm a bit conscious of this answer:

People need Buddhism when their current raft has sunk. If there is food on the table, a comfortable place to sleep, and they have no complaints about their daily routine, then our jobs as Buddhists is to rejoice in their success (mudita).
Anyhow, I don't claim to have a good solution for the problem-- how do we stay optimistic and positive or happy once we realize the muddle we are in, but at least the Mahayana version, provides a path towards how to find happiness-- by taking action to solve everyone's problems.

Do you agree with that and is that all there is to say on that subject?
Does it happen that Buddhism provides a new raft, or tells a person how to swim, but they still tend to be "depressed" e.g. unhappy and ineffective, as well as troubled or restless?
There's an illustration of "household joy" and "renunciation joy" at the end of this question. Assuming I can imagine a bit what "household joy" might be, would it be worth explaining that illustration of "renunciation joy"?
"Joy" seems to me to appear quite late in the "Ten Bulls" sequence -- relatively late compared to its being quite early in the Kimatthiyasutta -- i.e. it's the last, Return to society, which says, "I am ever blissful" etc. Is that significant? Or maybe it's earlier, like at the 5th of 6th stage.

@InstructionPointer Thanks for the edit. "Condition" is often used as a verb, though, e.g. "X is conditioned by Y".

The word for joy in AN 1.11 is "pamojja":

Pāmojja (p. 454) Pāmojja Pāmojja=pāmujja
Pāmujja (p. 453) Pāmujja Pāmujja (nt.) [grd. form. tr. pa+mud, see
  similar forms under pāmokkha] delight, joy, happiness; often combd
  with pīti. -- D i.72, 196; S iii.134; iv.78=351; v.156, 398; A iii.21;
  v.1 sq., 311 sq., 339, 349; Sn 256; Nett 29; DA i.217; Sdhp 167. See
  also pāmojja.

The connection here is to the five hindrances.
From SN 46.40:

“Bhikkhus, these five hindrances are makers of blindness, causing lack
  of vision, causing lack of knowledge, detrimental to wisdom, tending
  to vexation, leading away from Nibbāna. What five? The hindrance of
  sensual desire is a maker of blindness … The hindrance of ill will …
  The hindrance of sloth and torpor … The hindrance of restlessness and
  remorse … The hindrance of doubt is a maker of blindness … leading
  away from Nibbāna. These five hindrances are makers of blindness,
  causing lack of vision, causing lack of knowledge, detrimental to
  wisdom, tending to vexation, leading away from Nibbāna.

Ajahn Brahm commented on remorse (kukkucca) in this essay:

Remorse refers to a specific type of restlessness which is the kammic
  effect of one's misdeeds. The only way to overcome remorse, the
  restlessness of a bad conscience, is to purify one's virtue and become
  kind, wise and gentle. It is virtually impossible for the immoral or
  the self indulgent to make deep progress in meditation.

So, basically, the purpose or virtue or ethics is to remove remorse, in such a way that it will cause the arising of joy. From joy, it goes next to rapture (piti), then tranquility (passadha) then bliss (sukha).
Piti and sukha seems to refer to jhana.
Hence, the joy arising from the removal of remorse, is basically the effect of the removal of the hindrance of remorse i.e. kukkucca (which is tied to kammic effect of one's misdeeds), by inculcating virtues. Why is it called a hindrance? It's because it hinders progress in meditation, immersion and insight.
Once the hindrance of remorse is removed, then joy results. With this joy (and the removal of all other hindrances), one can progress more easily into jhana.
There is only one small problem here. The word for remorse in AN 1.11 is "vippatisara", while the word for remorse in SN 46.40 as a hindrance is "kukkucca". But the dictionary entry for kukkucca solves this problem:

Vippaṭisāra (p. 628) Vippaṭisāra Vippaṭisāra [vi+paṭisāra] bad
  conscience, remorse, regret, repentance Vin ii.250; D i.138; S
  iii.120, 125; iv.46; A iii.166, 197, 353; iv.69; J iv.12; v.88; Pug
  62; DhA iv.42; VvA 116; PvA 14, 60, 105, 152. -- a˚ no regret, no
  remorse A iii.46.
Kukkucca (p. 218) Kukkucca Kukkucca [kud -- kicca] 1. bad doing,
  misconduct, bad character. Def. kucchitaŋ kataŋ kukataŋ tassa bhāvo
  kukkuccaŋ Vism 470 & Bdhd 24; -- Various explanations in Nd2 on Sn
  1106=Dhs 1160, in its literal sense it is bad behaviour with hands and
  feet (hattha -- pada˚) J i.119=DA i.42 (in combn with ukkāsita &
  khipitasadda); hattha˚ alone J ii.142. -- 2. remorse, scruple, worry.
  In this sense often with vippaṭissāra; and in conn. w. uddhacca it is
  the fourth of the five nīvaraṇas (q. v.) Vin i.49; iv.70; D i.246; S
  i.99; M i.437; A i.134=Sn 1106; A i.282; Sn 925; Nd2 379; DhA iii.483;
  iv.88; Sdhp 459; Bdhd 96. -- na kiñci k˚ŋ na koci vippaṭissāreti "has
  nobody any remorse?" S iii.120=iv.46. The dispelling of scrupulousness
  is one of the duties and virtues of a muni: k˚ŋ vinodetuŋ A v.72; k.
  pahāya D i.71=A ii.210=Pug 59; chinnakukkucca (adj.) free from remorse
  M i.108; khīṇāsava k˚ -- vūpasanta S i.167=Sn 82. -- akukkucca (adj.)
  free from worry, having no remorse Sn 850. Kukkuccaŋ kurute (c. gen.)
  to be scrupulous about J i.377; kariŋsu DhA iv.88; cp. kukkuccaŋ
  āpajjati (expl. by sankati) J iii.66.

Let's agree that remorse would hinder joy. What I was asking is for example, once there is no remorse then does joy simply arise spontaneously (and even permanently), or should it be cultivated somehow as well (e.g. user12901's answer suggests it follows from anussati)? And what about other hindrances like uddhacca -- isn't uddhacca too a hindrance to joy ... and separate from remorse? Perhaps it's obvious how lack-of-remorse arises (i.e. the cause is sila) what causes lack-of-restlessness? Does it arise or is it developed? Is joy the cause or the consequence of a lack of restlessness?

Translators vary wildly on how they render the many pali words that describe happiness/joy. So the first thing you want to do is find out which pali word you're really asking about.
Here are my notes, comparing some of the words used for joy/happiness.
http://lucid24.org/tped/s/sukha/index.html
For AN 11.1, the word we're interested in is pamojja, a conjugated form of modati and mudita (the 3rd brahma vihara). Pīti and mudita are often used together in a compound word, and they have a very similar meaning, if not synonymous. Here is a concise yet comprehensive summary of pīti.
https://notesonthedhamma.blogspot.com/2019/07/the-miseducation-of-piti-in-theravada.html
7sb (the 7 awakening factors), often appear in a slightly different form in the suttas, not explicitly labeled bojjhanga, but clearly is the 7sb sequence. The AN 11.1 sequence is meditative, a causal sequence, just like 7sb, but also they're independent factors one can develop out of order, and simultaneously.
Mudita works very much like piti, it's a specific type of altruistic joy from appreciating virtuous qualities that lead to the end of dukkha. For example, you see someone get a promotion at their work, and you are happy for them. That's not mudita, in the EBT sense of the word.

Joy is important because it is how you turn satipatthana into samadhi, which is done by knowing that you are doing something good, not meritorious, but really good, skillful, the proper thing to end dukkha.
In the anussati like buddhanussati, it is this joy which is generated and this is how to get straight to nibanna. As usual, when it comes to knowledge, aryans are privileged over puthujjanas. Puthujjanas doubt (at best) the dhamma, the buddha, the sangha, sila and so on and they deeply hate satipattha since sati, right view, right thoughts (the dispassion-nekhamma part) is really the opposite of what they believe and as puthujjanas the least worst wrong doctrine they can create is the non-ill will, compassion, love stuff which is at best meritorious (ie when puthujjanas do not make the usual mistake that ''as long as I feel compassionate, I can do whatever I want and it will be good karma") but still not the non-dark non-bright karma... Puthujjanas will always lack the dispassion part, the nekhamma and even more over papanca, ideas, consciousness, birth, becoming (the usual stuff non-returners still love), which is the other half required to nibanna.
This is why doubt is the biggest fetter, then the next one is lack or too much energy (mostly lack).
Joy is always meritorious and it is what it means (or rather lack of regrets) to clean thoughts when the vitakka are tracked in satisampajanna.
In fact, cleaning the thoughts is really the first time somebody does an activity with good karma, unless there is dana to some arya because this is easier (when aryas are here). Then the continuation of this is cleaning the vedanas and sannas going from dirty sensual vedanas to whatever pleasant vedana  there is in jhanas.
But puthujjanas cling too much to thoughts to even want to change their bad thoughts into good thoughts, let alone be skilled at it.

Buddha taught joy as a step on the way to Nirvana.
If we take complete absolute "everything is wrong!" dukkha of a hell, and complete absolute "everything is just so" of Nirvana, then joy with its "this is right!" is somewhere half way.
Although some Zen and Vajrayana schools teach the suchness of Nirvana right away, the Buddha of Pali Canon taught gradual path, which involves approximating Nirvana step by step, rather than trying to skip all the way to the end. The Cow Sutta illustrates the principle of gradual training. If one does not get stable on previous state before moving up, one risks regressing even below where one started. So Nirvana is to be approximated step by step.
When something is wrong, we experience dukkha. When everything's wrong, we experience a lot of dukkha. The more wrongness - the more dukkha, and vice versa. As things get better, there's less dukkha.
Amount of wrongness we experience depends first of all on amount of disharmony we got ourselves in. Disharmony by definition is about clashes and conflict, and conflict means some right is challenged by some wrong. When our life is a mess, when we are messed up enough to create major trouble for ourselves, then disharmony is pretty much the norm, then conflict, clashes, wrongness is all around us, and correspondingly there's a lot of dukkha. So reducing dukkha starts with reducing objective real life trouble. Which basically means, reducing amount of conflict and disharmony we generate. Which basically corresponds to what the worldly people recognize as good ethics.
Once the external disharmony in our individual life is mostly eliminated, in the sense that things are now simple and there's no drama, what remains are internal sources of dukkha. To understand these we need to understand how our mind evaluates experience.
When we have a frame of reference in our mind, a coordinate system that defines some right and wrong, and when we use it to evaluate any experience, then we recognize some things as right and some as wrong, and thus we experience dukkha according to rules inherent to that frame of reference. When we switch frame of reference, the experience is now evaluated differently, so some other right/wrong comparison is made, and it generates a different amount of dukkha.
When our external life has little drama and things are simple, we can focus on purely internal dukkha. We start looking at our patterns of evaluation, to see what we label as right and wrong, and how we generate dukkha. We see that most of it comes from some ideas about ourselves, how our life is structured, where we are, where we should be, what we have, what we should have etc. Our goal on this stage is to genuinely reorientate our frame of reference in such way, that our experience is evaluated from the perspective of Dharma. In other words, we should stop worrying about being wrong by the worldly standards (not successful, not rich, not famous etc.) and instead be happy if our life is correct by the standards of Dharma. Which means NOT that we should live on the mountain in the monastery, but that we are no longer creating major trouble, that our ethics is strong, our mind is more or less stable etc.
So on this phase we both achieved some everyday harmony. and we convinced ourselves that we are on the right track and nothing else is principally missing, nothing else matters. As a result of this, our frame of reference used for evaluating our experience is not finding anything wrong. Everything is more or less right, both externally (our actual life is alright) and internally (our opinion about ourselves). So we finally experience a moment when everything is good. When we actually get to this moment, and we look at ourselves - what we experience at that very second is The Joy. It is that feeling that we finally got somewhere in your Buddhist practice, your life is not a mess, you have nothing to be guilty about in terms of ethics, and you managed to stop worrying about worldly standards and goals, so basically you are good. Finally! This is a good reason for joy, isn't it?
This joy is good in all respects. It gives you some reward for all your ethics, fixing life, simplifying it. It gives you something good that you can enjoy right here, so you don't have to run after worldly achievements. And it is an actually harmonious state that can be used as platform for further practice.
The game is not over yet, Nirvana is a much more refined state than this joy, and eventually you will see how much hidden dukkha you still have, but for now, this is some very good achievement. It is totally worth being cultivated, to wash away residual trouble-making patterns (both external and internal), and to stabilize on this level.
In some sense, this joyful state of harmony is like a basic version of Nirvana, similar in the sense of having less disharmony and more harmony, just not as deep as the real thing.

Piti - This is bliss due to absorption. This is what is discussed in Kimatthiyasutta. Following are some of the types:

Weak rapture only causes piloerection.
Short rapture evocates some thunder "from time to time".
Going down rapture explodes inside the body, like waves.
Exalting rapture "makes the body jump to the sky".
Fulfilling rapture seems to be a huge flood of a mountain stream.

When you experience this the body will sway, vibrate or even bounce when is it at reasonably intense.

Somanassa - this is an agreeable feeling like when one gets a present one likes or doing and activity one likes. A householder may do something agreeable to him without proper understanding that this results in pain in the future. He may get attached to possessions without knowing that there is misery to protect it from thieves and when it breaks or get lost, etc. The joy of renunciation is the fruits to recluseship like Jhana or other meditative attainments.
Pamojja, pamudita - this is a result of:

due to non-guilt by being virtuous - Kimatthiya Sutta, Cetanā’karaṇīyā Sutta
faith - Upanisā Sutta
wise attention - Das’uttara Sutta
The practice of the Ground of liberation - Vimutt’āyatana Sutta

listening to the Dharma
teaching the Dharma
reciting the Dharma
reflecting on the Dharma
meditation

beginning with moral virtue, sīla
would naturally experience non-guilt, avippaṭisāra
which leads to joy, pamudita
which leads to zest, pīti
which leads to a tranquil body, passadha,kāya
which brings happiness, sukha
which conduces to mental concentration, samādhi
which allows us to see true reality, yathā,bhūta
which leads to revulsion, and nibbidā
which results in the knowledge and vision of freedom. vimutti,ñāṇa,dassana

Nibbida by Piya Tan
Up to pamudita one can realise without meditation by means of what is mentioned above. Beyond this one needs to meditate. One can achieve pamudita through meditation also. These stages above need to be developed one after the other.
Joy (pīti) comes after Pamojja. This arises due to the suppression of the 5 hindrances by practising the 1st Jhana. When one person concentrates on one object exclusively there is sense withdrawal.

"There is the case where a monk — quite withdrawn from sensuality, withdrawn from unskillful qualities — enters and remains in the first jhana: rapture and pleasure born from withdrawal, accompanied by directed thought and evaluation. He permeates and pervades, suffuses and fills this very body with the rapture and pleasure born from withdrawal. There is nothing of his entire body unpervaded by rapture and pleasure born from withdrawal.

Jhana
One cannot choose joy over Dukkha as joy is subjected to Viparinama-dukkha.

Within the Buddhist sutras, dukkha is divided into three categories:

Dukkha-dukkha, the dukkha of painful experiences. This includes the physical and mental sufferings of birth, aging, illness, dying; distress from what is not desirable.
Viparinama-dukkha, the dukkha of pleasant or happy experiences changing to unpleasant when the causes and conditions that produced the pleasant experiences cease.
Sankhara-dukkha, the dukkha of conditioned experience. This includes "a basic unsatisfactoriness pervading all existence, all forms of life, because all forms of life are changing, impermanent and without any inner core or substance." On this level, the term indicates a lack of satisfaction, a sense that things never measure up to our expectations or standards.

Dukkha


text:
When and why did the color orange (and yellow and 'red') become associated with Buddhism?
Buddhist religious figures often wear orange robes.
And I think that orange is, also, associated with Buddhist religious buildings -- and other religious "stuff" (although this may be simply a coincidence that I have given meaning it does not have) -- things like fabric, prayer flags, and religious art in temples, and painting of religious structures and artistic adornments in religious structures.
When did the color orange become associated with Buddhism? Does this have any specific significance or meaning, and if so what is that meaning or significance?
Also, sometimes a yellow or red color is used instead in the a similar context. The yellow color often looks like just a use/weather faded orange, or a trick of photography and lighting, or a different shade of orange, that may not be meaningful. But the red color (as depicted below), however, seems to be an intentional choice.
Do either of these colors (yellow or red) these have different meanings than orange? And, when did these colors gain significance?
Is it simply derived from robes -- What is the meaning behind different robe colors? -- or is there more to it than that?

From here (Pixabay License - Free for commercial use - No attribution required).

From here (Pixabay License - Free for commercial use - No attribution required).

From here (Pixabay License - Free for commercial use - No attribution required).
Footnote Re Color Terminology
While this question, as edited, is not predominantly about robes as that has already been answered, the photographs above are primarily included to illustrate the colors that I am talking about as I am art terminology illiterate. I have used the primary color names orange, yellow and red because I have a thin vocabulary of colors and no art background (I haven't taken an art class since I was 13 years old, and I'm now more than three decades older than that). If that is not the proper name of the relevant colors feel free to correct me.

Comments about whether this question is a duplicate of the 'color of robes' question are moved to chat.

Thank you for the edit. I got busy at work and didn't have a chance to get back to this is quickly as I would have preferred.

The oldest Buddhist robe color we know of is yellow/orange. The original monks made their robes from discarded cloth found in rubbish and on cremation grounds. After washing, the robe-cloth was boiled with vegetable matter—leaves, roots and flowers—and often spices, which would turn the cloth some shade of orange. Hence the name, "saffron robe."
The tradition stuck and yellow/orange is now the color of choice for Theravada Buddhists in Southeast Asia, as opposed to a maroon color for Chinese and Tibetan monks. This has various reasons: first, the climate is very different,so the natural dyes of South-East Asia are not found in Tibet. The harsher climate also means that the traditional triple robe would offer little protection from the elements. The second reason is traditional robes expose the arms and shoulders, which was deemed unseeming in Chinese culture. So monks began to wear a long robe with sleeves that fastened in the front, similar to robes worn by Taoist scholars. The robes worn by Tibetan monks are derived from a mixture of the clothing "rules" from the traditional robes and Tibetan & Chinese "dress code".
Japanese and Korean Buddhist monks also wear robes styled after the clothing traditions of their culture. In Japanese culture, red is associated with power and blood, and yellow is associated with nature. Shinto priests predominantly wear black, so this became the most common color for Buddhist robes in Japan.

Am I wrong that robe colors are also intentionally and meaningfully used in Buddhist art, textile objects, structures like temples, etc., or is this just me jumping to conclusions when there is no relationship or it is just an accidental or coincidental point based upon the fact that you suggest that the robe colors simply involve widely available dyes found in discarded materials which are used for all sorts of things in the regions in question rather than being particularly Buddhist in character?

I believe those colors can generally be traced back to other influences than the dye colors available. The best example is the Tibetan Prayer Flag, which symbolises the elements. A significant other source for color symbolism is the association of color with the realms of the wheel of life. Also, I suppose you would not necessarily have the same natural colors available for painting or sculpture as the ones used for textile.


text:
Is it possible to attain stream-entry if one is only following "the 5 precepts"?
Is it possible to attain stream-entry if one is only following "the 5 precepts"?
Are there any suttas that seem to address this question?

If someone follows 5  Percepts strictly at bodily & mental level , that mighty warrior won't have to worry about anything as it will soon be realized that to this percept has that division,eg. ,not to kill means not to get angry ,which means not to rise body temperature & not to cling which means to be aware of temperature & it's cause ,which means to pin-point location within body from where temperature rise is first felt ,then to find reason in mind ,then to find lok-dharma(worldly concern) behind ,and so on...

"entering the stream" means you understood the main point of Buddhism and started applying that point to your entire life.
Which point? -- That you should stop creating causes for future dukkha.
Which causes create dukkha? -- Whatever leads to increase of conflict, increase of side-taking, increase of attachment, increase of obsession, increase of aversion, increase of ignorance - leads to increase of dukkha. Whatever leads to maintenance of conflict, maintenance of side-taking, maintenance of attachment, maintenance of obsession, maintenance of aversion, maintenance of ignorance - leads to maintenance of dukkha. Whatever leads to cessation of conflict, cessation of side-taking, cessation of attachment, cessation of obsession, cessation of aversion, cessation of ignorance - leads to cessation of dukkha.
The nature of dukkha is conflict, clash, mismatch - so if you are setting in motion forces that will set up conditions for arising of conflict, clash, mismatch - then that's what you will get. Once you get that - there's suffering. Correspondingly, when you are laying down causes in the here and now that are by nature conducive to harmony - then that's what you'll get. It is that simple. We create Dukkha by ourselves, so once we stop creating Dukkha we may still get some Dukkha created in the past, but eventually all the past causes will run out and the new causes will come to effect, and so gradually we will attain the natural harmonious state - Nirvana.
Once you really "get" this, once you see it clearly, then you stop acting like that, because you know it's for your own benefit. It's not just blindly following the five precepts. It's about getting the principle. Sometimes you may still make mistakes and create trouble - but as long as you got the principle in the overall - you will keep making progress.
This is when we say you are "in the stream" leading to Nirvana.

The Vera Sutta (AN 10.92) (quoted below) lists out the criteria for stream entry:

Five forms of fear and animosity are stilled (i.e. the effect of consistently and heedfully practising the five precepts)
Four factors of stream entry
Rightly seen and rightly ferreted out the noble method (i.e. understood dependent origination thoroughly through insight)

The four factors of stream entry are verified confidence (or experiential confidence) in the Buddha, Dhamma (the teaching) and Sangha, as well as, being endowed with the virtues appealing to the noble ones.
From the Vera Sutta (AN 10.92):

Then Anathapindika the householder went to the Blessed One and, on
  arrival, having bowed down to the Blessed One, sat to one side. As he
  was sitting there, the Blessed One said to him, "When, for a disciple
  of the noble ones, five forms of fear & animosity are stilled; when he
  is endowed with the four factors of stream-entry; and when, through
  discernment, he has rightly seen & rightly ferreted out the noble
  method, then if he wants he may state about himself: 'Hell is ended;
  animal wombs are ended; the state of the hungry shades is ended;
  states of deprivation, destitution, the bad bourns are ended! I am a
  stream-winner, steadfast, never again destined for states of woe,
  headed for self-awakening!'
"Now, which five forms of fear & animosity are stilled?
"When a person takes life, then with the taking of life as a requisite
  condition, he produces fear & animosity in the here & now, produces
  fear & animosity in future lives, experiences mental concomitants of
  pain & despair; but when he refrains from taking life, he neither
  produces fear & animosity in the here & now nor does he produce fear &
  animosity in future lives, nor does he experience mental concomitants
  of pain & despair: for one who refrains from taking life, that fear &
  animosity is thus stilled.
"When a person steals... engages in illicit sex... tells lies...
"When a person drinks distilled & fermented drinks that cause
  heedlessness, then with the drinking of distilled & fermented drinks
  that cause heedlessness as a requisite condition, he produces fear &
  animosity in the here & now, produces fear & animosity in future
  lives, experiences mental concomitants of pain & despair; but when he
  refrains from drinking distilled & fermented drinks that cause
  heedlessness, he neither produces fear & animosity in the here & now
  nor does he produce fear & animosity in future lives, nor does he
  experience mental concomitants of pain & despair: for one who refrains
  from drinking distilled & fermented drinks that cause heedlessness,
  that fear & animosity is thus stilled.
"These are the five forms of fear & animosity that are stilled.

Additional question from a comment:

OP: Would experiential faith in the Buddha, the Teaching, the Sangha and following the 5 precepts be enough for stream-entry?

No. All the criteria listed above are required.

Your answer nicely points out the criteria, but how do you discern that the noble method is dependent origination?

@m2015 I didn't quote the full sutta in this answer. If you look into it by clicking the link, you will see that it refers to dependent origination.

It says, "There is the case where a disciple of the noble ones notices: When this is, that is. From the arising of this comes the arising of that. When this isn't, that isn't. From the cessation of this comes the cessation of that.In other words: From ignorance, etc." So understanding co-arising and cessation. That's the subject of the four noble truths as well, the doctrine of the 12 nidanas is another example of the same, I wonder whether understanding four noble truths is enough -- perhaps that was enough for Kondañña in SN 56.11.

4NT is anyway included in verified confidence in the Dhamma.

In SN55.23 Mahānāma asks about stream-entry. After giving the standard answer of experiential confidence in the Buddha, Teaching and Sangha, Mahānāma persists and asks for more detail. Godha explains further:

“Godhā, a person must have four things for me to recognize them as a stream-enterer.
  What four?
  It’s when a noble disciple has experiential confidence in the Buddha …
  the teaching …
  the Saṅgha …
  And they have the ethical conduct loved by the noble ones … leading to immersion.
  When a person has these four things I recognize them as a stream-enterer.”

Ethical conduct is guided by precepts. Five is a great start. Undertaking more precepts has greater benefit and will deepen study.
Precepts are not about self-mortification. Precepts are guides for wholesome behavior. For example, the precept to not kill might lead one to consider being a vegetarian out of compassion for animals instead of gorging on steak.
Precepts aren't about scoring high on merit--they are just thought guides and references that allow us to examine our motivations with clarity and an open heart.
Therefore to answer your question...
Since the five precepts guide ethical conduct, you would still need the first three things. You would also need experiential faith in the Buddha, the Teaching and the Sangha.

Would experiential faith in the Buddha, the Teaching, the Sangha and following the 5 precepts be enough for stream-entry?

For me it would be, since I think like Godha. For you it might not be. Mahanama insisted further and both of them went to see the Buddha. Stream-entry is experiential faith, not liberation. For more details see AN7.15. The stream-enterer is at the start of a long and wonderful journey. May your path be swift and sure!
https://suttacentral.net/an7.15/en/sujato

Would experiential faith in the Buddha, the Teaching, the Sangha and following the 5 precepts be enough for stream-entry?
I'd like to say "yes" -- and that "experiential faith" is already a lot and must be enough to begin to work with.
What troubles me, about that answer, is that maybe there is something else important -- i.e. meeting or knowing an arya person.
Is it also necessary to to meet or know, interact with, to learn from, to try to help, an arya person? Or is that hard to say?
In the Buddha's day you probably wouldn't hear the dhamma except in person; so the case wouldn't arise (can you even have "experiential faith in the sangha", without meeting "the sangha"?); but now ...
This answer mentions ...

In fact, cleaning the thoughts is really the first time somebody does an activity with good karma, unless there is dana to some arya because this is easier (when aryas are here).

... which I can't deny from experience, and might be canonical.
And granted that, I wonder if that's universally/always another requirement.
A reason I'm not happy with that as an answer ("you need to meet an arya") is that it might lead the question, "how should I meet an arya?", and so on, and heading off to search for one instead of maybe accepting your own knowledge of the dhamma.
I mean it might be a reasonable desire, but it might be a craving and eventually a not-good kind of some guru-worship.
Maybe it's a good thing to think about, though -- people you know who are ethical, people you have known and learned from, or could know.

One can start with the 5 precepts but one has to go through the following milestones along the way:

beginning with moral virtue, sīla
would naturally experience non-guilt, avippaṭisāra
which leads to joy, pamudita
which leads to zest, pīti
which leads to a tranquil body, passadha,kāya
which brings happiness, sukha
which conduces to mental concentration, samādhi
which allows us to see true reality, yathā,bhūta
which leads to revulsion, and nibbidā
which results in the knowledge and vision of freedom. vimutti,ñāṇa,dassana

Nibbida by Piya Tan
Do to that one has to follow the threefold training (trisikkhā). Sila is only one of the threefold training (trisikkhā):

higher virtue (adhisīla-sikkhā)
higher mind (adhicitta-sikkhā)
higher wisdom (adhipaññā-sikkhā)


text:
In Dependent Co-Arising, is Dukkha a Sankhara, a Vedana, both, or none?
How should I understand dukkha (in all its amplitude and semantic complexity), if I want to see it through the lens of Dependent Co-Arising?
Where is it classified, apart from the final nidana?
Is it a sankhara? Is always something felt and, therefore, a feeling?
If dukkha is always something to be felt (as vedana), then does that mean that everytime the chain of conditionality gives rise to the final nidana, the chain "starts again" in vedana?
Thanks in advance for your time.
Kind regards!

All suffering is defined as attachment to the aggregates therefore all suffering is attachment (MN 56.11). Attachment is a sankhara.
The suttas (MN 1) say delight is the root of suffering (nandī dukkhassa mūlan ti). 'Delight' is a synonym for 'attachment' (MN 38 - delight in feelings is attachment - yā
 vedanāsu nandī tadupādānaṃ). 'Delight' & 'attachment' are 'sankhara'. Therefore, those dependently originated phenomena that grow from this 'root' of delight, such as becoming, birth, aging, death, sorrow, lamentation, pain, grief, despair & suffering are also 'sankhara'.
Explained from the viewpoint of 'vedana':

And why, bhikkhus, do you call it feeling? ‘It feels,’ bhikkhus,
  therefore it is called feeling. And what does it feel? It feels
  pleasure, it feels pain, it feels neither-pain-nor-pleasure. ‘It
  feels,’ bhikkhus, therefore it is called feeling.

And what, bhikkhus, is feeling? There are these six classes of
  feeling: feeling born of eye-contact, feeling born of ear-contact,
  feeling born of nose-contact, feeling born of tongue-contact, feeling
  born of body-contact, feeling born of mind-contact. This is called
  feeling.

Thus D.O. emphasizes vedana/feeling is born from sense contact at the 6th link rather than from aging & death (even though aging & death is also a type of sense contact but of the type called āyatanānaṃ paṭilābho).
While final dukkha includes 'feeling', what is felt is the pain caused by the sankharas, per MN 141:

And what is pain? Katamañcāvuso, dukkhaṃ? Physical pain, physical displeasure, the painful, unpleasant feeling that’s born from physical
  contact. Yaṃ kho, āvuso, kāyikaṃ dukkhaṃ kāyikaṃ asātaṃ
  kāyasamphassajaṃ dukkhaṃ asātaṃ vedayitaṃ, This is called pain. idaṃ
  vuccatāvuso: ‘dukkhaṃ’.
And what is sadness? Katamañcāvuso, domanassaṃ? Mental pain, mental displeasure, the painful, unpleasant feeling that’s born from mind
  contact. Yaṃ kho, āvuso, cetasikaṃ dukkhaṃ cetasikaṃ asātaṃ
  manosamphassajaṃ  dukkhaṃ asātaṃ vedayitaṃ, This is called sadness.
  idaṃ vuccatāvuso: ‘domanassaṃ’.

For example, when people grieve, they also experience physical & mental pain from the grieving. 'Grief' is a 'sankhara'.

Grief: The normal process of reacting to a loss. The loss may be physical (such as a death), social (such as divorce), or occupational
  (such as a job). Emotional reactions of grief can include anger,
  guilt, anxiety, sadness, and despair. Physical reactions of grief can
  include sleeping problems, changes in appetite, physical problems, or
  illness.

Hi! Thanks, again, for your answer. Couldn't grief be considered a mental vedana, in specific, an unpleasent one? Kind regards!

No. Grief is lamenting over the death or loss of something. Therefore grief is thinking & attachment and concocting views of self

Thanks! After pondering a lot about it, I think I understand the point. Kind regards!

Its not very difficult. Imagine your girlfriend with the boobs, hips, lips & buttocks leaves you for me, and starts having sexual intercourse with me. For you, it is like getting kicked very hard in the stomach. This kick in the stomach is pain; but it arises from your thinking about your beloved girlfriend fornicating with me. The pain is born from sankhara (mental proliferations of attachment). Kind regards

Plenty of other things than vedana are dukkha, so you cannot say that dukkha is just vedana.
The buddha says that whatever is anicca is dukkha, so the way to find dukkha is to find something anicca, and all the stuff in Dependent Co-Arising are anicca.
the buddha says that this is to be known about dukkha:

"'Stress should be known. The cause by which stress comes into play
  should be known. The diversity in stress should be known. The result
  of stress should be known. The cessation of stress should be known.
  The path of practice for the cessation of stress should be known.'
  Thus it has been said. In reference to what was it said?
Birth is stress, aging is stress, death is stress; sorrow,
  lamentation, pain, distress, & despair are stress; association with
  the unbeloved is stress; separation from the loved is stress; not
  getting what is wanted is stress. In short, the five
  clinging-aggregates are stress.
"And what is the cause by which stress comes into play? Craving is the
  cause by which stress comes into play.
"And what is the diversity in stress? There is major stress & minor,
  slowly fading & quickly fading. This is called the diversity in
  stress.
"And what is the result of stress? There are some cases in which a
  person overcome with pain, his mind exhausted, grieves, mourns,
  laments, beats his breast, & becomes bewildered. Or one overcome with
  pain, his mind exhausted, comes to search outside, 'Who knows a way or
  two to stop this pain?' I tell you, monks, that stress results either
  in bewilderment or in search. This is called the result of stress.
"And what is the cessation of stress? From the cessation of craving is
  the cessation of stress; and just this noble eightfold path — right
  view, right resolve, right speech, right action, right livelihood,
  right effort, right mindfulness, right concentration — is the path of
  practice leading to the cessation of stress.
"Now when a disciple of the noble ones discerns stress in this way,
  the cause by which stress comes into play in this way, the diversity
  of stress in this way, the result of stress in this way, the cessation
  of stress in this way, & the path of practice leading to the cessation
  of stress in this way, then he discerns this penetrative holy life as
  the cessation of stress.
"'Stress should be known. The cause by which stress comes into play...
  The diversity in stress... The result of stress... The cessation of
  stress... The path of practice for the cessation of stress should be
  known.' Thus it was said, and in reference to this was it said.

https://www.accesstoinsight.org/tipitaka/an/an06/an06.063.than.html
As usual dukkha arises from craving   and ceases from the cessation from craving like that

“One who craves for and delights in bodily form, craves for and
  delights in dukkha. One who craves for and delights in dukkha will not
  attain liberation from dukkha. In the same way one who craves for and
  delights in feeling … perception … formations … consciousness, craves
  for and delights in dukkha. One who craves for and delights in dukkha
  will not attain liberation from dukkha.

https://suttacentral.net/sa7/en/analayo
If you want to find dukkha, just make craving arise and puthujjanas already crave and delight so it won't be hard to find dukkha.
So when there is no delight, this happens

"There are ideas cognizable via the intellect — agreeable, pleasing,
  charming, endearing, fostering desire, enticing. If a monk does not
  relish them, welcome them, or remain fastened to them, then in him —
  not relishing them, not welcoming them, not remaining fastened to them
  — there arises no delight. From the cessation of delight, I tell you,
  comes the cessation of suffering and stress. {By this means, Punna,
  you are not far from this doctrine and discipline."
At that time the Blessed One said to the monks: “You should rightly
  examine the eye as impermanent. One who examines it like this is
  called ‘with right view’. Because of rightly contemplating it,
  disenchantment arises. Because of the arising of disenchantment, one
  is free from delight and free from lust. Because of being free from
  delight and lust, I say the mind is rightly liberated.
“In the same way [one should rightly examine] the ear … the nose … the
  tongue … the body … the mind … [up to] … one is free from delight and
  free from lust. Monks, because of being free from delight and lust, I
  say the mind is rightly liberated.

https://suttacentral.net/sa188/en/analayo
And the way to not have delight is this:

“In the same way you should give right attention to feeling …
  perception … formations … consciousness, contemplating consciousness
  as impermanent, knowing it as it really is. Why is that? One who gives
  right attention to consciousness, who contemplates consciousness as
  impermanent and knows it as it really is, will eradicate desire and
  lust in regard to consciousness. One who eradicates desire and lust in
  regard to consciousness, I say, liberates the mind.

https://suttacentral.net/sa2/en/analayo
which is done with ''diligence''

“To a type of monks like this I say that they [should] cultivate
  diligence. Why is that? As such a monk trains in the faculties and
  delights the mind accordingly, is endowed with the necessities of
  life, and associates with good friends, he will soon attain the
  destruction of the influxes, the influx-free liberation of the mind
  and liberation by wisdom, knowing here and now for himself and
  realizing that: ‘Birth for me has been eradicated, the holy life has
  been established, what had to be done has been done, I myself know
  that there will be no receiving of any further existence.’
“Why is that? In relation to forms cognized by the eye which he could
  crave for with delight and become defiled by attachment, on having
  seen them that monk does not delight in them, does not praise them, is
  not defiled by them, and is not established in the bondage of
  attachment. As he does not delight in them, not praise them, is not
  defiled by them, and is not established in [the bondage of]
  attachment, he diligently progresses in the appeasing of body and
  mind.
“With the mind fully established in peace without forgetfulness,
  always concentrated and single-minded, with boundless joy in the
  Dharma, but still attaining the foremost concentrative attainment, he
  will certainly not regress by following a form with the eye. In
  relation to [sounds cognized by] the ear … [odours cognized by] the
  nose … [flavours cognized by] the tongue … [tangibles cognized by] the
  body … mind-objects cognized by the mind it is also like this.”

https://suttacentral.net/sa212/en/analayo

I marked this answer up despite some puthujjana errors in it.

That's not constructive criticism, a prescriptive comment.

It is because the world and our experiences are conditioned pain arises.

Verse 278: "All conditioned phenomena are dukkha"; when one sees this with Insight-wisdom, one becomes weary of dukkha (i.e., the khandhas). This is the Path to Purity.

Also, conditioned existence leads to one type of dukkha

Dukkha-dukkha, the dukkha of painful experiences. This includes the physical and mental sufferings of birth, aging, illness, dying; distress from what is not desirable.
Viparinama-dukkha, the dukkha of pleasant or happy experiences changing to unpleasant when the causes and conditions that produced the pleasant experiences cease.
Sankhara-dukkha, the dukkha of conditioned experience. This includes "a basic unsatisfactoriness pervading all existence, all forms of life, because all forms of life are changing, impermanent and without any inner core or substance."[web 1] On this level, the term indicates a lack of satisfaction, a sense that things never measure up to our expectations or standards.

Dukkha
50 of the 52 cetasikas is Sankara

The Abhidhamma lists 52 kinds of cetasikas. One is feeling (vedanaa), another is perception (saññaa). The remaining 50 are grouped together under the term sa"nkhaaraa.

Here vedana is not Sankhara. But this leads to Dukkha.

pleasant feeling is pleasant when it persists, painful when it changes;
painful feeling is painful when it persists, pleasant when it changes;
neutral feeling is pleasant when there is knowledge of it, painful when there is no knowledge of it.

Cūla Vedalla Sutta
So dukkha arises due to vedana and sankhara hence in this angle of thinking it is neither. Because of vedana and sankhara, dukkha arises hence in this angle it can be considered dukkha.

In D.O.,

past and present sankhara is subjected to Sankhara-dukkha.
whatever Vedana has an element of dukkha (pleasant feeling is painful when it changes; painful feeling is painful when it persists; neutral feeling is painful when there is no knowledge of it.)


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Is Intention a sankhara?
I would like to know if the following premises are well stated and if their content is true. Also, I'd like to know if the conclusion/answer makes sense:
1) In the aggregates, 'sankhara' are kammic formations.
2) Kammic formations are any phenomena that have an underlying intention, and therefore create new kamma.
Question: Is intention (cetana) a sankhara?
Thanks in advance for your time.
Kind regards!

1) In the aggregates, 'sankhara' are kammic formations.

Yes. Sankhara creates future results. So karmaic results are due to Sankhara. Sankhara is like the seed which germinates into the result.

2) Kammic formations are any phenomena that have an underlying intention, and therefore create new kamma.

Intention decides which of the 121/89 mind states arise. Each mind has The 52 Mental Factors (cetasika). Out of the 52, 50 are Sankhara which results in conditioning future experiences which are also known as karmic results.

Question: Is intention (cetana) a sankhara?

Cetana is a part of sankhara, which is 1 part out of 50 mental factors. There are 49 other factors which are considered sankhara. Cetasika has sanna and vedana also, excluding these factors the rest is sankhara out of which Cetana is one factor. As cetana is a Universal mental factor this always arising with the rest of the mental factors pertaining to a mind-state.
Cetana is also part of the phenomenon triggered by contact, as all minds arise with contact. With each mind-state all factors pertaining to vedana, sanna and sankhara arise out of which cetana is one factor.
Centana decides the wholesomeness or unwholesomeness of the mind and sankhara creates. E.g. For a positive mind, 14 of the Unwholesome mental factors will be missing. These 14 are sankhara which gives bad results.

Hi! Thanks for your answer! What do you mean with your last statement? Is Cetana a composed phenomenom? What are its components? Kind regards!

See this chart.

I made some edits. See if it is clearer.

1) In the aggregates, 'sankhara' are kammic formations.

Mmmm... I wouldn't say that. All "kammic formations" are "sankhara" but not all sankhara are kammic formations. For example, a Buddha has sankhara aggregate operating (SN 22.85) but I wouldn't call it "kammic".

2) Kammic formations are any phenomena that have an underlying intention, and therefore create new kamma.

Mmm... yes & no, I suppose. While AN 6.63 says "kamma is "intention", I would highlight the term "underlying tendency" for it to be "kammic". An "underlying tendency" ("anusaya") is always deluded or ignorant.

Question: Is intention (cetana) a sankhara?

Yes, absolutely. There is at least one sutta where "sankhara khandha" is defined as "intention" ("cetana"), However, I personally would not adhere to this definition strictly, given it is too limited. All intentions are sankhara but not all sankhara are intentions.

Elsewhere you mentioned that Kamma is a mundane teaching, but as long as one isn't an arahant, one must necessarily be mindful of the law of kamma, right?

Yes. Absolutely.