...In 386, St John Chrysostom preached a sermon linking the date for Christmas to the date of the Annunciation. He does so in a way that suggests that this was already an established belief. The date of the Annunciation was based on a Jewish tradition that the world was created on March 25, or Nisan 15, according to the Jewish calendar. The Jews also believed that a great man would die on the same day as his conception. The early Christians (who were of course Jews) therefore concluded that Jesus had been conceived on March 25. This made it the date of the world’s creation, and the start of the world’s redemption (and therefore the new creation).
Take that and stuff it into your Saturnalia.