There's an old question about the star opens a wealth of other questions.
Firstly, the change BC/AD to BCE/CE. That's because Herod died in 4BC - but if he ordered the newborn killed, then Jesus was born in 4BC or earlier.
Secondly, we know from the Romans when Quirinius was Governor of Syria, and why a census was held: it was in 6AD, when the Romans fired Herod Archelaus for incompetence and took over Judea. Sounds like the lad's Bar-Mitzvah, Luke's got it wrong (again), so his birth's 7 or 6BC.
As it happens, the Chinese reported a supernova in July 7BCE...

So why Dec 25th? It's JC's fault - Julius Caesar, that is. He wanted his calendar running from Mar 25th, and that was the year-end, Lady Day, until they corrected his Calendar in 1754. He chose it for the Equinox, so the Solstice fell on Dec 25, and indeed that was certainly the case in 79CE, per Pliny the Elder, who stayed up to measure it. Of course, it's 21st now, just shows we lost another 4 days along the road.

Didn't end there, when they dropped 11 days from the calendar to get the Gregorian system working, the City rioted because they didn't want to pay their taxes early, and so the tax year got put back to April 5th.

So, we celebrate Winter Solstice, on the wrong day, in the wrong month, in the wrong year. The only good thing is that it gives you plenty of time to practice, and have a sporting chance of a barbie to celebrate.

That is, if the Horned Dancers don't get you...