Well knock me over with a feather.
The commonly-told explanation for the early Christians choosing the date of Christmas - that it was piggybacked onto a gift-giving Roman holiday Saturnalia, in and effort to woo pagans over to celebrating the birth of Jesus? Turns out it's likely not true, according to Biblical Archaeology Review. It's a good example of how hearing a plausible theory that explains some of the data can cause you to forget what you already know. I had known that the very earliest Christians didn't pay much attention to Christmas at all. Easter was the big deal, as it should be. And if you'd asked the question in the right way, I would have answered that over the next few centuries, the Church were concerned with distancing itself from pagan customs, not embracing them and co-opting them. That came much later, when it was making a more concerted effort to convert my ancestors in northern Europe. But I breezed right by those known facts because the Saturnalia (plus a few other pagan celebrations) theory sounded so plausible.
The most loudly touted theory about the origins of the Christmas date(s) is that it was borrowed from pagan celebrations. The Romans had their mid-winter Saturnalia festival in late December; barbarian peoples of northern and western Europe kept holidays at similar times. To top it off, in 274 C.E., the Roman emperor Aurelian established a feast of the birth of Sol Invictus (the Unconquered Sun), on December 25. Christmas, the argument goes, is really a spin-off from these pagan solar festivals. According to this theory, early Christians deliberately chose these dates to encourage the spread of Christmas and Christianity throughout the Roman world: If Christmas looked like a pagan holiday, more pagans would be open to both the holiday and the God whose birth it celebrated.(CWCID: First Things)
Despite its popularity today, this theory of Christmas’s origins has its problems. It is not found in any ancient Christian writings, for one thing. Christian authors of the time do note a connection between the solstice and Jesus’ birth: The church father Ambrose (c. 339–397), for example, described Christ as the true sun, who outshone the fallen gods of the old order. But early Christian writers never hint at any recent calendrical engineering; they clearly don’t think the date was chosen by the church. Rather they see the coincidence as a providential sign, as natural proof that God had selected Jesus over the false pagan gods.