Christmas is a season, not a day!
The Christmas Season, or Christmastide, begins on the evening of Christmas Eve (December 24) and ends on the Solemnity of the Baptism of the Lord (January 8). The Octave of Christmas (octave means eight) begins with Christmas day and ends after the Solemnity of Mary, Mother of God (January 1).
Then the liturgical calendar focuses on the next immediate Sunday, counting off days before and after it: Epiphany. Epiphany commemorates the recognition of Jesus as the Son of God by the Magi (and by extension, all nations).
The season of Christmas ends on Monday the Solemnity of the Baptism of the Lord (January 8), which signifies the purification of the world, through Christ himself.
Thanks Be to God and Boomer Sooner!
Day 2 of Christmastide and Feast of St. Stephen