The church year doesn’t begin in January, and the church’s liturgical calendar isn’t the same as the secular calendar.
The first Sunday of Advent is the beginning of the new liturgical year. The gospel reading switches over to a different gospel.
Some parishes may begin new programs at the beginning of Advent, or start parish council terms, or make other changes. You may recall that a couple years ago, when we switched over to the new translation of the Mass, that happened on the first Sunday of Advent, too. All that is because it’s the beginning of the new church year.