From the GIRM, via the USCCB website:
In all the dioceses of the United States of America, January 22 (or January 23, when January 22 falls on a Sunday) shall be observed as a particular day of penance for violations to the dignity of the human person committed through acts of abortion, and of prayer for the full restoration of the legal guarantee of the right to life. The Mass “For Peace and Justice” (no. 22 of the “Masses for Various Needs”) should be celebrated with violet vestments as an appropriate liturgical observance for this day.