Included in the psalm literature, of which there is not a little, there are many penitential psalms in which the penitent seeks forgiveness for his sins. (religion-online.org)
Both festivals are preceded by penitential seasons: Advent comes before Christmas and Lent, which recalls the forty days that Jesus spent fasting in the wilderness, comes before Easter. (religion-online.org)
The use of ashes in penitential rites has origins in the Old Testament, and the Church continued to use ashes in its liturgical practices for the same symbolic meanings of mourning, penance, and mortality: (firstthings.com)