THE BIRTHDATE OF JESUS
The date of Jesus’ birth is not known. Dionysius (1st century) is known for doing the historical math and arriving at a birth year around BC 12. Others disagreed.Generally, Jesus’ birth date is now placed around 4 BC, but there is nothing of theological or spiritual significance that hangs on this date. It was not a priority in the early church, and no writer of Scripture saw fit to include a date.
The early church associated birthday celebrations with the pagan gods. Early Christian writers (Irenaeus, 130–200; Tertullian, 155–240; Origen of Alexandria, 165–264) mocked Roman celebrations of birth anniversaries, dismissing them as “pagan” practices—a strong indication that Jesus’ birth was not marked with festivities at that place and time. Origen
(c.185-c.254) said it would be wrong to honor Christ in the same way Pharaoh and Herod were honored. Tertullian did not list it as a Christian holiday for sure.
When Jesus’ birthdate was discussed, the date would have been figured out from a tradition that martyrs died on the same date they were conceived. If Jesus died on 14 Nisan (March 25), he was conceived on a March 25, which meant he was born on December 25 if the timing was perfect.
Hippolytus' Commentary on Daniel (early 200s) claimed either March or December 25 as the date for Jesus' birth; Clement thought March 25 as the date of Jesus conception, thus 9 months before his birth and death.
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THE ROMAN INTERLUDE: DID CHRISTIANS JOIN A PAGAN HOLIDAY?
SATURNALIA: In the time that Jesus was born, Roman had been observing Saturnalia starting December 17 and generally lasting 6 days. It was a holiday in honor of Saturn, “the birthday of the unconquered sun,” and it was a party (to say the least) characterized by a lot of personal and societal chaos. It was a mix of good and bad for sure.
There seems to be little reason to think Christians chose December 25 to join or subvert a pagan holiday. The Jewish population from which Christianity emerged was quite good at establishing their own holidays, and their math was based on Jesus’ death date/conception date. Really, because the early church did not celebrate birthdays, the likelihood of Saturnalia influencing a Christmas celebration is small. The more likely candidate for potential overlap is the next one.
SOLIS INVICTI. “On December 25th, 274 AD, the Emperor Aurelian created a holiday called Dies Natalis Solis Invicti – the birthday of the Sun – officially elevating the Sun to the highest position among the gods.” This would be a better candidate for the melding of Christian and pagan holidays, but by the time December 25 becomes a Christian celebration, Solas Invicti was largely more of a cultural festival than a religious one. In addition, this means the holiday was created well after Hippolytus and others had claimed that day as a potential birth day.”