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.
Every Sunday is a Holy Day
Every Sunday, the Lord’s Day, is recognized as a holy day of obligation and Catholics are bound to participate in Mass on Sunday or at an anticipated Mass on Saturday in the late afternoon or evening.
What if a Holy Day falls on a Saturday or Monday?
In the diocese of the United States, whenever
falls on a Saturday or on a Monday, the precept to attend Mass is abrogated (removed). This action only applies to above days. It does NOT apply to December 8, Immaculate Conception. When either December 8 or December 25 falls on a Saturday or a Monday the faithful are obligated to attend the Mass for that day in addition to that for the Sunday which either precedes or follows it. Attending the
celebration of any one Mass cannot fulfill the obligation for both days.
What about December 8: Immaculate Conception of the Virgin Mary?
What about December 25: The Nativity of the Lord (Christmas)?
What about Ascenscion Thursday?