There are two separate obligations that must be fulfilled for the 4th Sunday of Advent (Sunday, Dec 24th) and Christmas (Monday, Dec 25th).
Given that Christmas falls the day after the 4th Sunday of Advent, the expectation can be confusing to some.
The 4th Sunday of Advent obligation can be fulfilled by going to Mass on Saturday evening (Dec. 23rd) or anytime on Sunday (Dec 24th).
The Christmas obligation can be fulfilled by going to Mass on Sunday evening (Dec. 24th) or anytime on Monday (Dec. 25th).
No matter how it is accomplished, the key is that the two obligations must be fulfilled separately. (NO DOUBLE DIPPING!)
ST. MARY MOTHER OF GOD MASS SCHEDULE
Sunday, Dec. 24th, 4th Sunday of Advent: 10:45 a.m.
Sunday, Dec. 24th, Christmas Eve: 4:00 p.m.
Monday, Dec. 25th, Christmas Day: 10:45 a.m.