Holy Days of Obligation are specific days in the Catholic Church calendar when Catholics are required to attend Mass. These days are significant for commemorating important events in the life of Jesus Christ, the Virgin Mary, and the saints. Typically, there are several Holy Days of Obligation each year, including Christmas and the Feast of the Assumption. When these days fall on a Sunday, the obligation to attend Mass is fulfilled by participating in the Sunday service. Observing these days helps strengthen the faith and community among Catholics.