Why did early Christians choose to celebrate Christ’s birthday on December 25th? Well not all did. To this day different Christian Churches celebrate Christmas on different days – the Orthodox Christmas is January 7th in our calendar, 25th December in theirs. However in the west the festival has been fixed at December 25th since the early Third Century. So why?
The world was believed to have been created on March 25th – Adam’s birthday. Jesus as “Second Adam” (see Pauline Epistles) was assumed to share this birthday initially. So March 25th was probably the original birthdate celebrated. Being a Prophet, he was also believed, in accordance with custom, to have been born and died (like Shakespeare) on the same day. Passover may well have fallen on March 25th, so the Crucifixion was then, so again, Jesus’ birthday by this logic!
All Christian antiquity (against all astronomical possibility) recognized the 25th of March as the actual day of Our Lord’s death. The opinion that the Incarnation also took place on that date is found in the pseudo-Cyprianic work De Pascha Computus, c. 240. It argues that the coming of Our Lord and His death must have coincided with the creation and fall of Adam. And since the world was created in spring, the Saviour was also conceived and died shortly after the equinox of spring
So the jump is from birth to conception – add nine months to March 25th, and lo! December 25th! All entirely based on Jewish contemporary thinking; if coceived not born on March 25th, Jesus was born on December 25th. Wot no Mithras???
Around 400CE St. Augustine wrote in On The Trinity
For He is believed to have been conceived on the 25th of March, upon which day also he suffered; so the womb of the Virgin, in which he was conceived, where no one of mortals was begotten, corresponds to the new grave in which he was buried, wherein was never man laid, neither before him nor since. But he was born, according to tradition, upon December the 25th.
So there, that is the real,and fairly odd to our minds reasoning that led Christmas Day to be on December 25th – nothing to do with pagan festivals upon that date, of which there were none.