Many Christians think that Christians celebrate Christ’s birth on December 25th because the church fathers appropriated the date of a pagan festival. Almost no one minds, except for a few groups on the fringes of American Evangelicalism, who seem to think that this makes Christmas itself a pagan festival. But it is perhaps interesting to know that the choice of December 25th is the result of attempts among the earliest Christians to figure out the date of Jesus’ birth based on calendrical calculations that had nothing to do with pagan festivals.
Rather, the pagan festival of the “Birth of the Unconquered Son” instituted by the Roman Emperor Aurelian on 25 December 274, was almost certainly an attempt to create a pagan alternative to a date that was already of some significance to Roman Christians. Thus the “pagan origins of Christmas” is a myth without historical substance.
The idea that the date was taken from the pagans goes back to two scholars from the late seventeenth and early eighteenth centuries. Paul Ernst Jablonski, a German Protestant, wished to show that the celebration of Christ’s birth on December 25th was one of the many “paganizations” of Christianity that the Church of the fourth century embraced, as one of many “degenerations” that transformed pure apostolic Christianity into Catholicism. Dom Jean Hardouin, a Benedictine monk, tried to show that the Catholic Church adopted pagan festivals for Christian purposes without paganizing the gospel.
In the Julian calendar, created in 45 B.C. under Julius Caesar, the winter solstice fell on December 25th, and it therefore seemed obvious to Jablonski and Hardouin that the day must have had a pagan significance before it had a Christian one. But in fact, the date had no religious significance in the Roman pagan festal calendar before Aurelian’s time, nor did the cult of the sun play a prominent role in Rome before him.
Go read it all and be ready the next time someone tries to foist the "pagan" canard on you.