December 25--the traditional day for the birth of Jesus.
But, to thank for it, we have some pretty wild assuptions from the early third century. The calculations of Hippolytus of Rome (a Christian) satisfied him that March 25, AD 29, was the date of the crucifixion. You might wonder what it has to do with the birth of Jesus. Well, Hippolytus believed that crucifixion day was in synchrony with the day when Mary conceived Jesus--so, by that token, March 25 was conception day too!
The idea of perfect synchrony didn’t end there, either. Hippolytus fixed the nativity nine months after Mary conceived Jesus--and not a day more or less! So he arrived at the day December 25. Just like that.
His calculations drew on a 16-year cycle of the moon--he started from his day and went backwards with it and got March 25. Crucifixion day. But the real fact is that there's no such thing as a 16-year lunar cycle--it's really 19 years long! The 16-year cycle was bad astronomy--bogus.
Not knowing any better, the church in Rome embraced his March 25. (Somebody had proposed it before, actually--but the idea fell by the wayside.)
In the meantime, other churches disagreed--preferring March 25 to be the day of the nativity itself (Christmas Day).
Not only that, but Christians applied holy synchrony to other things in the Bible. For these early Christians, creation began on March 25. And so did the fall of Adam and Eve!
Hippolytus said that Jesus lived exactly thirty years and was born in 1 BC. Biblicists today, though, say 6 BC was a more likely date.
Recap: Hippolytus fouled up thanks to the faulty astronomical data he had to work with. His date for Jesus’ birthday, December 25, grew popular a century later, anyway--and competed with an official Roman sun festival.
23 Was the god-man Osiris born December 25