The near universal celebration of Christmas on December 25 dates to the early fourth century. It is commonly said that this date was chosen to counter the birthday of the pagan “Invincible Sun” on that date. Personally, I do not find that explanation very convincing.
Saint Hippolytus of Rome’s Commentary on Daniel 4:23 (dated A.D. 203), gives December 25 as Christ’s birth date. Although some think this text is an interpolation, it is a very early one. Since celebration of the Invincible Sun’s birth was not instituted at Rome until the reign of Emperor Aurelian (A.D. 270-275), the earlier existence of the Christian date makes a borrowing from pagan practice unlikely.
So why would Christians have picked December 25? Luke’s Gospel tells us that the births of Jesus and John the Baptist were separated by six months, and that John the Baptist was conceived after his father Zacharias saw a vision in the Temple and was struck dumb. The popular stories of the “Infancy Gospel of James” (early 100s) also say that Zacharias was a high priest (which he was not), implying that his vision happened on the Jewish Day of Atonement (Yom Kippur). That feast comes mostly in late September and probably did so in the second century too.
Ancient Christians could then have calculated nine months to the birthday of John the Baptist (late June) and then six months more to the birthday of Jesus (late December). Since the ancients believed June 24 was the longest day of the year and December 25 the shortest, settling on those as the exact dates would have been natural. Saint Bede (700s) wrote that John the Baptist preached that he had to decrease and Christ had to increase, and that is exactly what the daylight does, beginning on their respective birthdays.
–Fr. Augustine Thompson, O.P.
originally published as “Welcoming the Light of the World” in our Mission West newsletter, Vol. VII, Issue II, 2014