One of the things that makes the Roman Catholic Church distinct from other Christian denominations is that we have days throughout the year– in addition to Sundays– when we’re obligated to attend Mass. Holy days are important feast days in the life of the Catholic community on which, in addition to Sundays, Catholics (who are above the age of reason and who are not sick) are obligated to participate in Mass according to the precepts of the Church. They celebrate an important mystery of the Catholic faith. The Catechism of the Catholic Church explains holy days of obligation this way:
On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.
Family needs or important social service can legitimately excuse from the obligation of Sunday rest. The faithful should see to it that legitimate excuses do not lead to habits prejudicial to religion, family life, and health [CCC 2185].
The Code of Canon Law spells out when the holy days of obligation are on the Church’s universal calendar:
Can. 1246 §1. Sunday, on which by apostolic tradition the paschal mystery is celebrated, must be observed in the universal Church as the primordial holy day of obligation. Also to be observed are the day of the Nativity of Our Lord Jesus Christ, the Epiphany, the Ascension and the Most Holy Body and Blood of Christ, Holy Mary Mother of God and her Immaculate Conception and Assumption, Saint Joseph, the Apostles Saints Peter and Paul, and finally, All Saints.
In Canada, we only have 2 Holy Days of Obligation that are not transferred to Sunday’s Mass. They are Christmas and the Solemnity of the Mother of God on January 1. Without grave reasons, missing Masses on these days incurs the penalty of mortal sins.
For more reference, please check the following resources:
Holy Days | The Catholic Herald
Catholic Holy Days of Obligation | BeginningCatholic.com