When did you start using AD verses BC dating?
Christian bishops decided a date of the birth of Jesus the Nazorean, which became accepted. The monk who proposed the year botched the date - as according to the gospel of Matthew it was in the reign of Herod who died in 4 BC. According to Luke it was after 6 AD when Quirinius became governor of Syria. We just don't know.
The actual date was originally set at 6 January, but with the difficulty of suppressing the popular festival of the Saturnalia, Christmas was shifted on top of it to 25 December. The Orthodox Christian churches still retain 6 January.