Christmas is a season… that starts on Christmas Day. You can think of the season of Christmas as complementary to the season of Advent: during Advent we anticipate what during Christmas we celebrate.
You’ve heard the phrase “the twelve days of Christmas”? Those are the twelve days between Christmas Day and Epiphany. But the liturgical season of Christmas lasts all the way to the feast of the Baptism of the Lord, a week later. This feast marks the beginning of Jesus’ public ministry as an adult, as described in the gospels of Matthew, Mark, and Luke, and so it’s a suitable closing for the season of Christmas.