Holy Days of Obligation are feast days that honor the mysteries of the Lord, the Virgin Mary, and the saints.  On Holy Days of Obligation the faithful are required to attend Mass and to avoid (if possible) unnecessary work. The observance of Holy Days of Obligation is part of the Sunday Duty.
 
The Catholic Church in the United States currently celebrates six Holy Days of Obligation. (Not included in a list of Holy Days of Obligation is any feast celebrated on a Sunday, such as Easter, and thus falling under the normal Sunday Duty.)
 
1. January 1=The Solemnity of Mary, the Mother of God begins the year. On this day, the faithful are reminded of the role that the Blessed Virgin played in the plan of our salvation.
 
2.  The Ascension of Our Lord, which occurred 40 days after Christ rose from the dead on Easter Sunday, is the final act of redemption that Christ began on Good Friday. On this day, the risen Christ, in the sight of His apostles, ascended bodily into Heaven. Readings: Deuteronomy 4:32-34, 39-40; Psalm 33:4-5, 6, 9, 18-19, 20, 22; Romans 8:14-17; Matthew 28:16-20
 
3.  August 15-The Solemnity of the Assumption of the Blessed Virgin Mary commemorates the death of Mary and her bodily assumption into Heaven.
 
4.  November 1-All Saints is the Christian tradition of celebrating the martyrdom of saints on a common feast in order to ensure that all martyrs, known and unknown, are properly honored.
 
5. Decenber 8-The Solemnity of the Immaculate Conception, celebrates, not the conception of Christ (a common misconception), but the conception of the Blessed Virgin Mary in the womb of Saint Anne; and nine months later, on September 8, the Nativity of the Blessed Virgin Mary is celebrated.
 
6.  December 25-Christmas (the combination of Christ and Mass) is the feast of the Nativity of Our Lord and Savior Jesus Christ. It is the last Holy Day of Obligation in the liturgical year.  Christmas is second in importance in the liturgical calendar only to Easter.