The Church imposes on us the Obligation to attend and participate in Sunday Mass and Mass on Holy Days of Obligation. They both carry the same weight, or obligation. Holy Days of Obligation are Solemnities that celebrate particular mysteries of the Faith that are recognized to be so important that they necessitate our presence at Mass.
Complete List of Holy Days of Obligation:
January 1 - The solemnity of Mary, Mother of God
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
If January 1, August 15, or November 1 fall on a Saturday or on a Monday, the obligation for Mass attendance is lifted.
However, when December 8, or December 25 fall on a Saturday or a Monday, the obligation for Mass remains. Further, the Sunday obligation for Mass also remains and you are Obligated to attend Mass for BOTH the Solemnity and the Sunday.
*please note, in the Archdiocese of Mobile, the Solemnity of the Ascension is moved to the Seventh Sunday of Easter, elsewhere this Solemnity may remain on Thursday of the Sixth Week of Easter, where is remains a Holy Day of Obligation.