Stands to reason: This is when the sun starts coming back. Who wouldn't want to celebrate?
Very few of the trimmings of Christmas are specifically Christian. Fir trees, holly, mistletoe, fat beardy men in fur-coats, reindeer- not many of any of these were to be found in the neighbourhood of Bethlehem in the good old days of Caesar Augustus.
The early Church worked on the sound principle "if you can't beat 'em join 'em". Finding they couldn't stop people decking their halls with boughs of holly, the fathers co-opted the festival by declaring it the birthday of their own Sun-god.
Come on in, Jesus, and meet the gang. The guy in the mask is Horus and that's Dionysus presiding over the punch-bowl and the Japanese lady in the shiny dress is Amaterasu.....