This is the time of year when we are told that Christmas is really a pagan holiday that was taken over by the Christians in ancient Rome in order to replace paganism with their own beliefs. Atheists love this and are happy to promote this idea because it reduces the historical certainty of the entry of Christ into our world, and erodes the credibility of Christianity as a whole if it's just paganism transformed into something slightly different. Some Christians also believe that the origins of Christmas are pagan, following in the footsteps of the old Puritans who hated celebrations of any kind, but I hope that if they investigate they will realise that nothing could be further from the truth.
There are two major arguments: that Christmas somehow copied the Roman feast week of Saturnalia, celebrated December 17-23, or that Christmas replaced the birth of the Roman Sun god, that had been celebrated on December 25. Both are ridiculous, as you shall see.
Let's start with Saturnalia, in the words of Fr Longenecker, who has a certain way with them.
When trying to solve the mystery of the relationship between Christmas and the Saturnalia we have to consider not only the similarities, but the differences. The Saturnalia was celebrated from December 17 – 23. Okay that’s pretty close to the December 25 date for Christmas–but if they were copying the Saturnalia, why didn’t the early Christians celebrate the Nativity of Christ on December 17? At the Saturnalia they had a feast. Good. Christians had a feast too. The Romans gave each other gifts as part of the celebration. There’s a match. Christians did too. However, the Romans also wore silly hats, got drunk, danced naked in the streets, propped up the statue of Saturn on a couch to observe the revelries, reversed roles between slaves and masters, and put green drapes around their doorways. None of those fun activities are part of Christmas.And then there's the Roman celebration of the birth of their sun god on December 25. Would be pretty damning, if it weren't for the fact that there is no evidence that this feast was celebrated by the Romans before A.D. 360. It might even be that December 25 was chosen by them for the birth of their sun god in an attempt to supplant Christmas, which by that time had grown enormously and was no longer persecuted by the Romans, causing more that a bit of consternation by those who believed that the old Roman gods needed to be appeased in order for Rome to regain her strength and vigour.
The most glaring difference is in the meaning of the celebration itself. If there were some sort of link with the birth of Christ you would expect that the meaning of the Saturnalia might have something to do with the coming of light in the dark time of the year or the birth of new life in the midst of the cold and dark. The Saturnalia has none of those themes [...]
So, why is Christmas celebrated on December 25? Fr Longenecker again:
In 386 St John Chrysostom preached a sermon linking the date for Christmas to the date of the Annunciation. He does so in a way that suggests that this was already an established belief. The date of the Annunciation was based on a Jewish tradition that the world was created on March 25. The Jews also believed that a great man would die on the same day as his conception. The date for Jesus Christ’s death was Nisan 15 according to the Jewish calendar, or March 25 according to the Roman calendar. The early Christians (who were of course Jews) concluded that Jesus was therefore conceived on March 25. Thus the day of the world’s creation, and the day of the world’s redemption (and therefore the beginning of the new creation) was March 25.Nine months after March 25, the conception of Our Lord, is of course, December 25, which is why Christmas is celebrated on December 25.
I'll leave you to discover why Fr Longenecker includes Frodo Baggins in his post title.
Read more: Christmas, Pagan Romans and Frodo Baggins ~ Fr. Dwight Longenecker
UPDATE: With the related question, Was Easter Originally A Pagan Holiday? Short answer - No.