There is no evidence for the observance of Christmas Day on December 25 before 300 AD. In fact, the earliest record of December 25 as a church festival is in the Philocalean Calendar (which was copied in 354 AD, but represents the practice of the Church of Rome in 336 AD). One reason given for this day being chosen is that heathens would have their festival of the sun on December 25, and so in an effort to contradict this festival, Christians decide that they would gather on this day to celebrate the birth of the Son of Righteousness.
(C.F. Pfeiffer, H.F. Vos, & J. Rea, Whcliffe Bible Dictionary, 2008)