According to the United States Conference of Catholic Bishops, the Holy Days of Obligation for American Catholics are all Sundays as well as:
- January 1, the solemnity of Mary, Mother of God
- Thursday of the Sixth Week of Easter, the solemnity of the Ascension (unless moved to the 7th Sunday of Easter)
- August 15, the solemnity of the Assumption of the Blessed Virgin Mary
- November 1, the solemnity of All Saints
- December 8, the solemnity of the Immaculate Conception
- December 25, the solemnity of the Nativity of Our Lord Jesus Christ
There are other Holy Days of Obligation, such as Easter, which are also celebrated, but because they fall on a Sunday they are already included in the usual Sunday obligation.
According to the Catechism of the Catholic Church, every Sunday is a Holy Day of Obligation and all Catholics are bound to participate in the Holy Mass, unless they are unable to for a serious reason.
Christians gathered each Sunday in the early days of the Church to celebrate the death, burial and resurrection of Jesus Christ. It was the most important day of the week, the "first of all days, the first of all feasts". According to the Catechism, "The celebration of Sunday observes the moral commandment inscribed by nature in the human heart to render to God an outward, visible, public, and regular worship 'as a sign of his universal beneficence to all.' Sunday worship fulfills the moral command of the Old Covenant, taking up its rhythm and spirit in the weekly celebration of the Creator and Redeemer of his people." There are many additional reasons why Sundays are incredibly important in the life of every Christian, which you can read about in Part 3, Section 2 of the Catechism. Catholics who do not fulfill the Sunday Mass obligation commit grave sin which requires confession to enter back into a state of grace.
Caveats:
- Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated.
- Holy Days of Obligation may vary from country to country. In addition, Holy Days of Obligation vary for Eastern Rite Catholics.