Common knowledge has it that many years ago, some Pope picked December 25 in order to eclipse Pagan celebrations. I myself believed this for a while, but it turns out this particular theory lacks any early witnesses to it, which is enough to kill it for me. Besides, the true story is weirder, which is another point in its favor.
Back in 200AD, Tertullian reported the date of the crucifixion as March 25, a calculation that seems to have been widely accepted. This, of course, is precisely 9 months before December 25. The idea is that Jesus’ death and his conception must have been on the same day. As Augustine put it, “[Jesus] is believed to have been conceived on the 25th of March, upon which day also he suffered; so the womb of the Virgin, in which he was conceived, where no one of mortals was begotten, corresponds to the new grave in which he was buried, wherein was never man laid, neither before him nor since. But he was born, according to tradition, upon December the 25th.”
The Eastern church used much the same method, but arrived at January 6 instead because they were using a different calendar. Armenians still celebrate Christmas on this date. This way of thinking is odd to our modern minds, which are much more comfortable with the idea of picking dates to steal other people’s holidays, but “early”1 Christians would have been more influenced by ancient Jewish notions that great events could be expected almost cyclically, at the same of year.
1 We don’t see references to Christmas’s date until the fourth century. Compared to us, these are indeed early Christians, but keep in mind that you yourself are seperated from America’s founding by just over 200 years. If a man in 300AD is an early Christian, then you are an early American2, and so are your descendants 100 years from now.
2 Unless, this being the internet, you’re not an American at all. Just go with it.