What do Christians mean by "holy"? What is the meaning of the word holy from a Christian viewpoint?
I observed that the term is used for

the gods, especially the Holy God (Father) and Holy Ghost;
persons, such as the holy apostles, Holy St. Florian, etc.;
buildings like churches;
organisations (the holy church);
relics and fetishes, such as the Holy Coat of Trier;
cities, such as the holy city (this title is used for both Rome and Jerusalem).

Since holiness is nothing which can be identified with a scientific instrument, what is it? Is it just an attribute, awarded by a higher priest, which you then believe in? But what is it? Do you have to believe, that it can, for example, have healing effects? Does it just mean that you have to treat something with extra respect?
This is what I found in an encyclopedia:

Heilig, von Heil, also soviel wie in seiner Vollkommenheit nicht nur noch unverletzt, sondern auch unverletzlich, unantastbar, dann soviel wie schlechthin gut, sittlich vollkommen, makellos. Seine Wurzeln hat dieser Begriff teils im römischen Kultus, wo er das dem gemeinen Gebrauch Entzogene, höhern Zwecken Gewidmete (sacer, sanctus), teils im Alttestamentlichen, wo der Ausdruck (kadosch), von Gott ausgesagt, dessen Unterschiedenheit von allem Irdischen, seine Unvergleichlichkeit und Erhabenheit, von Irdischem ausgesagt, dessen Zugehörigkeit zu Gott, Gottgeweihtheit bedeutet. Vgl. Baudissin, Studien zur Religionsgeschichte, Bd. 2 (Leipz. 1878); Issel, Der Begriff der Heiligkeit im Neuen Testament (Leiden 1887).
Meyers Großes Konversationslexikon.

For those of you who don’t speak German, I’ll try a translation:

Holy, from ‘intact, complete’, not only undamaged, but invulnerable, untouchable, but also completely good, ethical and morally perfect. The roots of the term reach to the Roman cult, where it means “taken away from common usage”, used for higher purpose (sacer, sanctus), partly in the Old Testament, where the term (kadosch), spoken of God, means his difference from anything terrestrial, his uncomparabliness and loftiness, and means “belonging to God” or antlared (? maybe not. -else: holiness, which is tautological. - remark by me, u.u.). Vgl. Baudissin, Studien zur Religionsgeschichte, Bd. 2 (Leipz. 1878); Issel, Der Begriff der Heiligkeit im Neuen Testament (Leiden 1887).

Maybe we can agree that a building can’t be morally good. Nor can bones, or cities, or weekdays.
Isn’t it fetishism to pray to dead objects made by men, to bones without spirit and clothes?
Or is holiness something like bacteria, which spread from God to a Saint, from the Saint to his clothes, from the clothes to a prayer? Is there evidence in the new Testament that touching such things is helpful, is something you should do?
The progression in your list is actually somewhat (although not exactly) representative of the decreasing level of weight carried by the word.

Let's start with this: Only God is holy.

Holy means to be set apart, entirely-other. Untouched by any imperfection. Unmarred by any fault. God is the only one that matches these criteria. And boy is he Holy. The angels in heaven cry out in eternal accord that he is thrice holy. Holy. Holy. Holy.

After that things go down hill fast. Absolutely nothing else exists or ever will exist apart from God that is holy in the way that he is.

And yet there is one crazy exception: Those who through faith in Christ are adopted to be joint heirs with Christ and inherit his very holiness. Over and over through the Old and New testaments we see God calling his chosen people to "be holy, as I am holy." This is an absurd thing to ask of a degenerate created being -- except that he also makes us this way by covering us with his own holiness. At the cost of the life of His own holy son God provided a way to make un-holy people holy.

Some more interesting powerful uses of the word holy involve things like the temple (both the tent tabernacle and the permanent temple) where the sanctuary was holy because it was dedicated to a certain use, but the inner chamber was the _holy of holies because that is where Gods very presence would dwell.

Moving on down the line, the word has some common usages that only involve being set apart or dedicated for a certain use. Church buildings are only holy not in that they are physically sacred but that they are reserved for certain uses ... they are dedicated to the work of God through his body the church.

Cities such as Jerusalem can be considered holy in the sense that they play a special role or enjoy a unique identity or history.