Catholic Holy Days of Obligation are the most important feast days of the entire Church year. On each Holy Day, all Catholics are expected to attend mass to celebrate and rejoice on these special occasions.
All Holy Days listed are for the Catholic Church in the United States. A number of Holy Days that are observed by the Universal Church are celebrated on Sundays and are therefore no longer considered Holy Days of obligation (although all Catholics still celebrate these feasts by attending mass on Sundays). Also of note, the Ascension of Jesus is considered a holy day in the United States in only a select few dioceses. Most elect to celebrate the Ascension the Sunday after its traditional date.
The Solemnity of Mary
This feast celebrates Mary as the Mother of Jesus. It is fitting and deliberate that this holy day is observed in the middle of Christmastime, when Christ's birth is at the forefront of our minds. On the Solemnity of Mary, Catholics celebrate the selfless love of Mary to freely accept God's will to be the "God-bearer."
The roots of this celebration date back to the 500s, but the Solemnity of Mary was not made an official holy day on January 1st until 1974.
On the feast of the Assumption, Catholics celebrate the assumption of Mary the Mother of God's body and soul into heaven. The Catechism states the "Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians." The Assumption, a dogma of the church as declared by Pope Pius XII in 1950, was first referenced in the early 4th century and is seen as a compliment and completion to Mary as the Immaculate Conception.
All Saints Day
All Saints Day can be traced back all the way to the 300s as a remembrance of all known and unknown saints. In the early Church, this feast rose out of the celebration of martyred saints who were very large in number during the days of harsh religious persecution by the Roman Empire. Since the 700s, All Saints Day has been celebrated on November 1st.
The Immaculate Conception
The feast of the Immaculate Conception celebrates the conception of Mary the Mother of God as being without the stain of original sin. This feast is often confused with the conception of Jesus (which is observed on the Annunciation). The Catholic Church believes that because Jesus was fully God and thus without sin, He must have been born of a woman also without sin. Celebrations of Mary's conception began in the seventh and eight centuries, and its feast day was eventually set as December 8th in 1854 by Pope Pius the IX.
Christmas, which celebrates the birth of Jesus Christ in Bethlehem, is second in importance in the Church calendar to only Easter. Christmas was not one of the feast of the early Church, which generally celebrated the deaths of saints. It was not until the mid fourth century that this feast began to spread throughout the Christian world.