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. Because Jesus died on Friday, March 25, the early Christians (who were of course Jews) therefore concluded that Jesus had been conceived on March 25. This made it the date not only of the world’s creation, but also the dawn of the world’s redemption (and therefore the new creation).
So this also determines the date of Christmas (not some goofy idea that the early Christians were embracing pagan solstice festivals). It’s easy. If the Lord Jesus Christ was conceived on March 25, then he was born nine months later on December 25. The date for Christmas is therefore determined by the date of the Annunciation–Ladyday.
J.R.R. Tolkien understood this, and so he has Frodo Baggins destroy the ring of power in the fires of Mt Doom on March 25–thus marking the dawn of a new age. That’s why today is a Tolkien reading day for fans of the Hobbit creator.
Furthermore, in the Middle Ages, Catholic society celebrated New Years’ Day not on January 1, but March 25–Lady Day–the first day of our world’s redemption.
So Happy New Year, Happy Ladyday, and long live Frodo Baggins!