Holy days of obligation upheld by Latin Catholics require parishioners to attend mass to celebrate the festivities unless they sick or are under the age of reason. The tradition for the holy days of obligation is detailed in the Canon 1246.
Holy days of obligation include the weekly Sunday mass, which is required of those in the Latin Catholic faith and include special holidays. These holidays differ from country to country. The United States Catholic Church celebrates 6 special holy days of obligation throughout the year, which are the Solemnity of Mary (January 1), the Ascension of Our Lord (seventh Sunday of Easter), the Assumption of the Blessed Virgin Mary (August 15), All Saint's Day (November 1), the Immaculate Conception of the Blessed Virgin Mary (December 8) and Christmas or the Nativity of Our Lord (December 25).
When the dates for these holy days fall on either a Monday or a Sunday then parishioners are not required to participate unless they choose to. This rule was agreed upon and decided during the National Conference of Catholic Bishops in 1991 as were the 6 holy days of obligation. The National Conference of Catholic Bishops is an event where bishops gather to review beliefs and teachings for the church.