With Christmas Eve on the 24th, it makes sense that Christmas is on the 25th.

He was probably born on February 30th

The romans had to keep the peace between the Roman aristocracy who worshipped the pagan gods and the ever growing Christian population.

That would on the celebrations on the solstices would protest and disrupt the pagan rituals and parties.
So the emperor ( too many beers to look up which one might have justinious ) but anyway the emperor decided to tell the Christians that they had discovered documents that showed that Jesus had been born on said date and had died on said date.

Seriously who the fuck was going to check ? So now everyone would be happy with their own celebrations and Rome had one less war to fight.

But on a side note the theory that the Romans created Jesus in order to control the lower class citizens has come in part from this action.
Just saying that if the romans made this up then what else did they create especially when you consider that the New Testament wasn’t written until 300-400 years after Jesus had supposedly lived and died.?

Tradition.

It was coopted by Chritians to inculcate pagans and make it easier for them to accept the new religion. Its a common strategy. In Bahia Brazil. the saints and saintly celebrations were equated with the local dieties. It served two purposes. 1) indigenous peoples could still practice their own religion but look like they ate converting. and 2) it made it easier to accept the new religion when in reality they had no choice. This happens all around the world wherever colonialism happens.

Like many Christian beliefs, they took from the pagans. The December 25th date was a pagan celebration of Saturn, the Roman god of agriculture. Doing so allowed them to convert Romans more easily.