Why is this day called Ash Wednesday?
Because on this day the Catholic Church blesses ashes and puts them on the foreheads of the faithful, saying, "Remember, man, that you are dust, and unto dust shall you return" (Gen. iii. 19DR, NAB).
Why are the ashes blessed?
1. That all who receive them with a contrite heart may be preserved in soul and body.
2. That God may give them contrition, and pardon their sins.
3. That He may grant them all they humbly ask for, particularly the grace to do penance, and the reward promised to the truly penitent.
Why are the faithful sprinkled with ashes?
The sprinkling with ashes was always a public sign of penance; as such God enjoined it upon the Israelites (Jer. xxv. 34DR, NAB). David sprinkled ashes on his bread (Ps. ci. 10DR, Psalm 102:10 NAB). The Ninivites (Jonas iii. 6DR, NAB), Judith (Jud. ix. 1DR), Mardochai (Esther iv. 1 DR, NAB), Job xlii. 6DR, NAB), and others, did penance in sackcloth and ashes.
[The following prayer is from a pre-Vatican II source and so is only of historical interest.]
To show the spirit of penance and to move God to mercy, the Church, at the Introit of the Mass, uses the following words:
"You have mercy upon all, O Lord, and hate none of the things which You have made, and [overlook] the sins of men for the sake of repentance, and sparing them, for You are the Lord our God" (Wis. xi. 24, 25DR, NAB). "Have mercy on me, O God, for my soul trusts in You." Glory be to the Father, etc. [For the text of the "Glory Be" and other traditional rosary prayers see Prayers of the Catholic Rosary on our site.]
See Hebrews 9:13, Matthew 11:21, Luke 10:13 in the New American Bible. See also CCC 1430 which puts interior penance ahead of external signs of repentance, such as ashes.
Goffine's Devout Instructions, Benzinger Brothers, NY. (1896) p. 87.