Everyone knows it’s because synchretistic Christians swiped the Roman Saturnalia, right? Guess again. William J. Tighe deals with “Calculating Christmas” for Touchstone Magazine. You don’t have to accept everything in the article to find it interesting.
[T]he pagan feast which the Emperor Aurelian instituted on that date in the year 274 was not only an effort to use the winter solstice to make a political statement, but also almost certainly an attempt to give a pagan significance to a date already of importance to Roman Christians.