We’ve received a few questions about this coming Sunday, regarding Mass obligations. As you know, Sunday will be the Fourth Sunday of Advent; Christmas is the very next day.
For those who are wondering, yes, you should attend Mass twice: 1) on Saturday night or Sunday morning to fulfil your Sunday obligation, and 2) on Sunday night or Monday morning for Christmas.
The reason is clear when you consider that there are two different celebrations: the Mass for the Fourth Sunday of Advent, and the Mass for The Nativity of the Lord. Therefore, attending one Mass on Sunday evening does not fulfill both your Sunday and Christmas obligations.