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) unnecessary work.  The observance of Holy Days of Obligation is part of the Sunday Duty, the first of the Precepts of the Church, and are listed in Canon 1246 of the 1983 Code of Canon Law.  Holy Days of Obligation are days other than Sundays on which Catholics are required to participate in the Mass, the primary form of worship.  Any feast celebrated on a Sunday, such as Easter, falls under normal Sunday Duty and thus isn't included in a list of Holy Days of Obligation.