On Sundays and other holy days of obligation, the faithful are obliged to participate in Mass. In so far as possible they are also to abstain from those works and affairs which hinder the worship to be rendered to God, the joy proper to the Lord's day, or the suitable relaxation of mind and body. (Canon 1247) At their November 1991 meeting, the United States Conference of Catholic Bishops decided that whenever January 1, the Solemnity of Mary Mother of God, or August 15, the feast of the Assumption of the Blessed Virgin, or November 1, the Feast of All Saints, falls on a Saturday or a Monday, the obligation to attend Mass is lifted.

Holy Days of Obligation

All Sundays of the year
Solemnity of Mary, Mother of God - January 1
The Ascension of Our Lord - 40 days after Easter (May 11/12)
The Assumption of the Blessed Virgin Mary - August 15
All Saint's Day - November 1
The Immaculate Conception of the Blessed Virgin Mary - December 8
Christmas, the Nativity of Our Lord - December 25