Holy Days

In the Catholic Church, Holy Days of Obligation are the days on which the faithful are obliged to participate in the Mass. They have also abstained from unnecessary works, "which hinder the worship to be rendered to God, the joy proper to the Lord’s day, or the suitable relaxation of mind and body." (Canon 1247 of the Code of Canon Law)

The list of Days of Obligation includes all Sundays in all countries. Some feasts, e.g. Easter, are always celebrated on Sundays, so they are always obligatory. The number of additional Days of Obligation varies between countries, from one in Hong-Kong to ten in the Vatican.

The number of Days of Obligation varies from year to year because the precept to attend Mass is lifted (abrogated), any of the following days: 

• January 1: Solemnity of Mary, Mother of God
• August 15: Feast of the Assumption
• November 1: Solemnity of All Saints

Special Dates of Prayer
Apart from days of obligation in the Catholic Church there are also often designated additional Special Days of Prayer, in which the faithful pray for some important intentions chosen by the Church. In those days the faithful are usually not obliged to attend Mass, but they are asked to observe these days in their devotions or private prayer.