system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the Gregorian Calender (Common Era) uses the birth of Christ as a reference for year numbering, why isn't Christmas celebrated on 1st of January?
gpt
The Gregorian calendar (1582) has nothing to do with year numbering.  It was just a small revision to the Julian calendar (45 BC) in order to account for the fact that a year wasn't *exactly* 365.25 days.  In other words, the calendar was set in place before Jesus was even born.  We had the same amount of days in each month as the emperors of Rome had.

Christmas is on December 25 because the Romans wanted the pagans to become Christian, so they sorta set Christmas up on the same day as the pagans' solstice festivals, to make it easier for them.  This was sometime in the 300s or 400s AD.  But the AD system wasn't invented until 525 AD (*after* Rome fell), when Dionysis Exiguus determined that Jesus was born in 1 AD (he was off a few years though).  The date of Jesus's birth was already "settled" by then...just not the year.

You are assuming that the religious holidays would have been set at the same time as the year numbering system (and at the same time as the calendar was set).  They were all different times.