One of the reason Christianity was able to spread so rapidly is they were able to adapt it to fit existing calendars and schedules. Did the birth of Christ occur....most assuredly it did. Was it December 25th? Most likely not. But folks already celebrated a holiday then, so you just adapt the date. It was easy peasy for folks to remember.

An old priest once told me, "its not how you get to God, but that you get to him." And that appears to have been the early Church's philosophy as well.