Origins of Ash Wednesday There has been a long standing association between ashes and repentance within Christianity stretching all the way back to Tertullian not to mention various grief filled moments of repentance found in the Old Testament. Ash Wednesday marks the beginning of Lent a period of fasting, inner reflection and repentance within the church calendar.

For a long time (until about the 8th century), however, the practice of sprinkling ashes or applying an ashen cross to the forehead as a mark of repentance was not used on the first day of lent. 

Why did this change? Was it a cultural shift? Was it an idea from the papacy or other church authority? Where did this particular practice come from? In essence then what is the origin of the Ash Wednesday ritual traditions?
cf. What Are the Origins of Ash Wednesday and the Use of Ashes? by Fr. William Saunders | Catholic Culture. In part


  The early Church continued the usage of ashes for the same symbolic
  reasons. In his book, De Poenitentia, Tertullian (c. 160-220)
  prescribed that the penitent must "live without joy in the roughness
  of sackcloth and the squalor of ashes." Eusebius (260-340), the famous
  early Church historian, recounted in his The History of the Church how
  an apostate named Natalis came to Pope Zephyrinus clothed in sackcloth
  and ashes begging forgiveness. Also during this time, for those who
  were required to do public penance, the priest sprinkled ashes on the
  head of the person leaving confession.
  
  In the Middle Ages (at least by the time of the eighth century), those
  who were about to die were laid on the ground on top of sackcloth
  sprinkled with ashes. The priest would bless the dying person with
  holy water, saying, "Remember that thou art dust and to dust thou
  shalt return." After the sprinkling, the priest asked, "Art thou
  content with sackcloth and ashes in testimony of thy penance before
  the Lord in the day of judgment?" To which the dying person replied,
  "I am content." In all of these examples, the symbolism of mourning,
  mortality and penance is clear.
  
  Eventually, the use of ashes was adapted to mark the beginning of
  Lent, the 40-day preparation period (not including Sundays) for
  Easter. The ritual for the "Day of Ashes" is found in the earliest
  editions of the Gregorian Sacramentary, which dates at least to the
  eighth century. About the year 1000, an Anglo-Saxon priest named
  Aelfric preached: "We read in the books, both in the Old Law and in
  the New, that the men who repented of their sins bestrewed themselves
  with ashes and clothed their bodies with sackcloth. Now let us do this
  little at the beginning of our Lent that we strew ashes upon our heads
  to signify that we ought to repent of our sins during the Lenten
  fast." As an aside, Aelfric reinforced his point by then telling of a
  man who refused to go to Church on Ash Wednesday and receive ashes;
  the man was killed a few days later in a boar hunt. Since this time,
  the Church has used ashes to mark the beginning of the penitential
  season of Lent, when we remember our mortality and mourn for our sins.


Please see also Ash Wednesday | New Advent.