The Internet will tell you that Christians borrowed the date from the Romans, who had a sun festival at that time. Romans did party at that time of year, but Christians had already chosen December as Christ’s birthday decades before. But why the 25th?
The key to the puzzle is figuring out when Christians thought Jesus was conceived. Most early Christians believed that the angel Gabriel appeared to Mary during the Jewish festival of Passover. This is also the time of year when Jesus was crucified, some 30 years later. Ancient Christians liked symmetry. It made sense to them that Jesus was conceived on the same date that he was killed. (This same logic was also applied to Moses’ birth and death dates).
December 25th was also appealing because it is around the time when the days begin to get longer than the nights. To Christians this made perfect sense—Jesus is the light of the world, so of course He would be born just as light was overcoming the long nights.
To this day, scholars say there is no way to prove which exact day or month Jesus was born. But the poetic logic of December 25th does have the advantage of giving us a way of lighting up the night on the darkest days of the year, a blessing in itself.
—adapted from Rev. Stephen Milton Blog -The United Church