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.