According to the Code of Canon Law, Sunday, the day we celebrate the resurrection of our Lord, is always observed as the foremost holy day of obligation for the universal Church. (The obligation involved is simply the duty to attend Mass on that day.) The Code also lists ten other holy days of obligation: Christmas; the Epiphany; the Ascension of our Lord; the Solemnity of Corpus Christi (the Most Holy Body and Blood of Christ); the Solemnity of Mary, Mother of God; Mary’s Immaculate Conception; her Assumption; the Solemnity of St. Joseph; the Solemnity of St. Peter and St. Paul; and All Saints Day. The Code notes that the conference of bishops can reduce the number of holy days of obligation or transfer them to Sunday with the approval of the Holy Father. (Confer the Code of Canon Law, #1246.)
In the United States, the Epiphany is transferred to the Sunday after January 1, and Corpus Christi is transferred to the second Sunday after Pentecost. At their November, 1991 meeting, the National Conference of Catholic Bishops in the United States decided to retain as holy days of obligation the Solemnity of Mary, Mother of God (January 1), Ascension Thursday (40 days after Easter), the Assumption of Mary (August 15), All Saints Day (November 1), the Immaculate Conception (December 8), and Christmas (December 25). However, whenever the Solemnity of Mary, Mother of God; the Assumption; or All Saints Day falls on a Saturday or on a Monday, the obligation to attend Mass is dispensed, meaning that the day is still a “holy day" but a person is not required to attend Mass. For example, if Christmas falls on a Saturday, the obligation remains to attend Mass; on the other hand, if the Solemnity of Mary, Mother of God (January 1) also falls on a Saturday, it remains a holy day but without the obligation to attend Mass. The Vatican confirmed this decision on July 4, 1992, and it became effective on January 1, 1993.
Nevertheless, we should not forget the importance of these holy days, whether or not there is the “legal" obligation to attend Mass. The Constitution on the Sacred Liturgy of the Second Vatican Council stated, “Thus recalling the mysteries of the redemption, [the Church] opens up to the faithful the riches of her Lord’s powers and merits, so that these are in some way made present for all time; the faithful lay hold of them and are filled with saving grace" (#102). Therefore, the importance of attending Mass on Sunday or any other holy day is not simply because of an obligation, but why it is an obligation. Our lives are so busy, and we face so many distractions. We could lose sight of God or become numb to His presence. Maybe we do have to sacrifice to attend Mass by rearranging our schedule or suffering some inconvenience to the normal course of life. So what? Our cherishing the mysteries of our salvation should take precedence over the exigencies of living in this world. Remember at the Last Supper, Jesus reminded the apostles that while they live in the world, they are not of this world (John 17:13-19). The holy days help us to remember the same. Therefore, we must pause to ponder, celebrate, and live the mystery of salvation by marking each Sunday, these special holy events, and the lives of those who are exemplars of faith with the offering of the Holy Mass.