Merry Dies Natalis Solis Invicti!
At least that's what you would be hearing (minus the Merry) if you happened to be around on December 25, 221 AD. The Romans had thought that the Solstice occurred on the 25th of December, and eventually Emperor Elagabalus decided that it would be a fine day to celebrate a Festival of the Unconquerable Sun. It died out for a while but then came back big under Aurelian in the later third century.
Shift over to the Christians, who had spent two hundred years focusing on the death of Jesus, not the birth. Finally, in the mid third century, Christianity decided that it was about time to settle on a date for the birth of The Savior. March 28, April 2, and November 18 were all bandied about with still no official decision save in the East, where January 6 was settled on in spite of the very strong Biblical evidence that Jesus couldn't have been born any time between November and February.
Since nobody really had a solid case for the exact day of Jesus's birth, it made sense to designate a day that would most help spread the influence of the Church. At the time (the late 200s and early 300s), the big Roman deity was the sun, whose big celebratory day we just saw was December 25, which also happened to be nine months after one of the equally arbitrarily arrived at dates for Jesus's conception (one of the justifications for which actually was that it falls nine months before Christmas, and so must have been the date of the conception, thus the date of the Annunciation fixed the date of Christmas, which then fixed the date of the Annunciation). So, December 25 it was, the decision being finalized in 435.
So, if you can see the sun where you are, tip your cap to it on what used to be its special day and maybe open a parasol and stand furtively under it to make it feel good about itself. Then go inside, and have a great day with the people you love.
- Count Dolby von Luckner