Holy days are like Sundays in that Catholics must attend Mass, and if possible, refrain from unnecessary servile work. 

In the United States, these holy days of obligation are

January 1: The Feast of Mary, the Mother of God
40 days after Easter Sunday: Ascension Thursday
August 15: Assumption of Mary into heaven
November 1: All Saints’ Day
December 8: The Feast of the Immaculate Conception
December 25: Christmas, the Nativity of Our Lord