From Whose Christmas Is It Anyway?
….during the last years of paganism, the cult of Sol remained so popular that the Church Fathers could only neutralize its celebration on the [traditional] winter solstice of December 25th by setting the birthday of Christ on that very same day. In other words, they snatched the day and, sooner rather than later, Christ trumped Sol.
This view is now almost universally accepted; but is it true?
….there is absolutely no evidence to show that the Games of the Sun founded by Aurelian ever took place on December 25th. On the contrary, no feast day for Sol is mentioned on that day until 80 years later in the Calendar of 354 and, subsequently, in 362 by Julian the Apostate in his Oration to King Helios (the Sun).
In short, while the winter solstice on or around the 25th of December was well established in the Roman imperial calendar, there is no evidence that a religious celebration of Sol on that day antedates the celebration of Christmas, and none that indicates that Aurelian had a hand in its institution.
There is a real possibility that the day was not dedicated to Sol until after the bishop of Rome first celebrated Christmas on that date in 336 AD — a pagan reaction to a Christian feast, perhaps, rather than vice versa.
Sol Invictus and Christmas explains why the earliest Christians did not celebrate Christmas, quoting Origen:
Not one from all the saints is found to have celebrated a festive day or a great feast on the day of his birth. No one is found to have had joy on the day of the birth of his son or daughter. Only sinners rejoice over this kind of birthday….the saints not only do not celebrate a festival on their birth days, but, filled with the Holy Spirit, they curse the day.
Calculating Christmas suggests how Christians arrived at the date of December 25 for the birth of Jesus.