All Sundays are obligatory for the faithful to participate in the Mass.  In addition, following are the Holy Days of Obligation for the Calendar Year:

January 1, the solemnity of Mary, Mother of God
Thursday of the Sixth Week of Easter, the solemnity of the Ascension
August 15, the solemnity of the Assumption of the Blessed Virgin Mary
November 1, the solemnity of All Saints
December 8, the solemnity of the Immaculate Conception
December 25, the solemnity of the Nativity of Our Lord Jesus Christ