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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.