Atheists, historians, Orthodox agree: Christians, like most subcultures, are very insular, and aren’t really paying attention to whatever intellectual fad has captured the popular imagination. But we live in Internet Times, so communicating anything you can learn from books is really only of the moment if you can communicate that same thing less well, but using hyperlinks.
In this particular case, the date for Christmas came up in another forum. (A Slack, since you didn’t ask.) To save myself the trouble of typing this out again, should the occasion arise, I’m posting it here on the Internet, where I can be sure it will last forever. (Things on the internet are permanent, right?)
Pagan festivals had zero influence on the date for Christmas.
No, but seriously: if you’re wondering why we celebrate Christmas on December 25th, it is based on the date for the Crucifixion. Tradition holds that Christ was crucified on the date that, in English, is identified as March 25th. (It’s actually a day in a month called “Nisan” in another language, but when you translate it into our calendar, it’s March 25th.)
(Another side note, here: the ancients, and especially Christians, didn’t really care about birthdays. This is a Germanic custom, like Christmas trees, that is charming and fun, but also wholly outside the scope of traditional & historical Christianity.)
According to an ancient tradition, a perfect life, or the life of a perfect man, begins and ends on the same day. Since the Crucifixion happened on March 25th, then Christ must have been conceived on the same day. No one knew for sure, and there are ancient notes that indicate that few people thought Christ was actually born in December. But the date wasn’t chosen to celebrate a birth, but to anchor the commemoration in a yearly cycle of commemorations.
If you check the dates of other births and conceptions on the church calendar, you’ll see that those dates are all off from a “perfect” 9 months by just 1 day. For example, the Orthodox & Roman Catholics commemorate the conception of the Theotokos / Mary by her mother, “the righteous Anna,” on December 9th, and commemorate the birth of the Theotokos / Mary on September 8th, 9 months less 1 day later.
Similarly, the conception by St. Elizabeth of the “Prophet, Forerunner, and Baptist” John, and his birth are separated on the church calendar by 9 months plus 1 day.
But the date for this event, the Crucifixion, is why the Annunciation (where a messenger from God tells the Theotokos / Mary that she is going to bear the Christ) is celebrated on March 25th, too - because of that ancient belief about a perfect life beginning and ending on the same day. And it’s why, a perfect 9 months later, we celebrate the Liturgy of the “Nativity in the Flesh of Our Lord, God, and Savior, Jesus Christ” - the Christ Mass, or, in the vernacular, “Christmas”.
Please note that none of the foregoing is meant to claim (or even imply) that Christians did not borrow, in some cases wholesale, from pagan religions. Of course they did. Lots of those practices and customs are really cool. Other practices participate in the underlying truth (“logos”) of the human experience in meaningful and important ways. Frequently, those practices are both cool and meaningful.
But borrowing a date from someone else because you’re jealous of them?
Ain’t nobody got time for that.