Christmas day is Monday, Dec. 25, which follows the Fourth Sunday of Advent. There have been some questions with regard to the obligations of these holy days. 

There are two distinct obligations which must be met by attending two Masses. While two obligations must indeed be fulfilled by two Masses, it is not necessary that one of these Masses be a Mass of Fourth Sunday of Advent and the other be a Christmas (Eve/Night/Dawn/Day) Mass. One can fulfill the two obligations also by attending Christmas Eve Mass (or Night Mass if before midnight) on the Sunday and a Christmas Night/Dawn/Day Mass on the Monday from midnight onward.

It is not the liturgical celebration that matters but the fact of being at Mass. Obviously the separate celebration of the Fourth Sunday of Advent and the Nativity of the Lord would be the preferred observance. The participation at one Mass only will not fulfill both obligations.

— Office of Divine Worship