Holy days of obligation are feast days on which Catholics are required to attend Mass and to avoid (to the extent that they are able) servile work. The observance of Holy Days of Obligation is part of the Sunday Duty, the first of the Precepts of the Church. There are currently ten Holy Days of Obligation in the Latin Rite of the Catholic Church; in the United States, only six Holy Days of Obligation are observed.
In the United States, the Catholic Church currently celebrates the six Holy Days of Obligation listed below. (Any feast celebrated on a Sunday, such as Easter, falls under our normal Sunday Duty and thus isn't included in a list of Holy Days of Obligation.)
While the 1983 Code of Canon Law for the Latin Rite of the Catholic Church mandates ten Holy Days of Obligation, the bishops' conference of each country can reduce that number. In the United States, two of the other four Holy Days of Obligation—Epiphany and Corpus Christi—have been moved to Sunday, while the obligation to attend Mass on the other two days, the Solemnity of Saint Joseph, Husband of the Blessed Virgin Mary, and the Solemnity of Saints Peter and Paul, Apostles, has simply been removed.
The Latin Rite of the Catholic Church begins the year by celebrating the Solemnity of Mary, the Mother of God. On this day, we are reminded of the role that the Blessed Virgin played in the plan of our salvation. Christ's Birth at Christmas, celebrated just a week before, and was made possible by Mary's fiat: "Be it done unto me according to Thy word."
The Ascension of Our Lord, which occurred 40 days after Jesus Christ rose from the dead on Easter Sunday, is the final act of our redemption that Christ began on Good Friday. On this day, the risen Christ, in the sight of His apostles, ascended bodily into Heaven.
The Solemnity of the Assumption of the Blessed Virgin Mary is a very old feast of the Church, celebrated universally by the sixth century. It commemorates the death of Mary and her bodily assumption into Heaven, before her body could begin to decay—a foretaste of our own bodily resurrection at the end of time.
All Saints Day is a surprisingly old feast. It arose out of the Christian tradition of celebrating the martyrdom of saints on the anniversary of their martyrdom. When martyrdoms increased during the persecutions of the late Roman Empire, local dioceses instituted a common feast day in order to ensure that all martyrs, known and unknown were properly honored. The practice eventually spread to the universal Church.
The Solemnity of the Immaculate Conception, in its oldest form, goes back to the seventh century, when churches in the East began celebrating the Feast of the Conception of Saint Anne, the mother of Mary. In other words, this feast 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, we celebrate the Nativity of the Blessed Virgin Mary.
The word Christmas derives from the combination of Christ and Mass; it is the feast of the Nativity of Our Lord and Savior Jesus Christ. The last holy day of obligation in the year, Christmas is second in importance in the liturgical calendar only to Easter.