The 4th Sunday of Advent and Christmas are both holy days of obligation. These are two distinct obligations. The obligation to attend Mass for Sunday can be fulfilled either Saturday evening at 4:00 pm or Sunday at 10:00 am. The obligation to attend Mass for Christmas can be fulfilled at Christmas Eve 4:00 pm, 6:30 pm, 10 pm, or on Christmas day at 10:00 am.
One must go to Mass twice. One may fulfill both obligations by going to Mass Sunday morning for Sunday and Sunday evening for Christmas.
The Feast of Mary, Mother of God, January 1, 2018 is not a Holy Day of Obligation this year.
We look forward to celebrating the seasons of Advent and Christmas with you and your family.