On holy days of obligation, Catholics are obliged to participate in Mass.  Every Sunday is a holy day of obligation, as are six other days throughout the year.  In the United States, these holy days of obligation are:

January 1:  The Solemnity of Mary, the Mother of God
August 15:  Solemnity of the Assumption of the Blessed Virgin Mary
November 1:  Solemnity of All Saints
December 8:  The Solemnity of the Immaculate Conception
December 25:  The Solemnity of the Nativity of Our Lord