Holy Days of Obligation are feast days on which Catholics are required to attend Mass, just as on Sundays. In the United States, there are six: (1) Solemnity of Mary, Mother of God — January 1. (2) Ascension of the Lord — 40 days after Easter (transferred to Sunday in some dioceses). (3) Assumption of the Blessed Virgin Mary — August 15. (4) All Saints' Day — November 1. (5) Immaculate Conception of Mary — December 8 (patron feast of the USA). (6) Christmas — December 25. When certain Holy Days fall on a Saturday or Monday, the obligation may be lifted by the local bishop. Missing Mass on a Holy Day of Obligation without a serious reason is considered a mortal sin.