That was God’s confirmation and celebration an event as it occurred in real time - and the event only occurred once. It had to be celebrated as it occurred - for at that time the only true God was incarnated in flesh in His only begotten Son.
It was NOT the celebration of a past event that had occurred on the same day. There WAS such a celebration of a single DAY that occurred annually for the Jews, and that was the Passover - so they’d never forget it and its significance.
Never in the first centuries of the church did they ever celebrate the birth of Christ. It was a practice - as the original poster noted - that was adopted around 300 AD when many pagan practices were adopted by formal Christianity - most brought in in the time of Constantine who wanted to satisfy the populace when he declared Christianity the national religion and pagans would have protested if they had not been able to keep their current practices. That’s just factual history.
The New Testament was written as late as 80 AD approximately, and there is not a word in any of the writings in the NT that speak of any practice of celebrating Christ’s birth. If it were practiced by the early saints, or was important, it would have been in Paul, Luke’s, Peter’s or James writings or another saints writings - but it is not.
But this disparity in views should not affect our fellowship in Christ. It is NOT a central matter, only a peripheral one.
True, the Bible doesn’t mandate celebrating Christmas. The same is probably true for Epiphany, Lent, Easter, Pentacost and Thanksgiving. Are Christians who observe these occasions committing sin?
Anyway, happy holidays!