Fr. Dwight Longenecker has a carefully researched and documented article based on Scripture, Jewish tradition and early Christian writings, stating that December 25 is indeed the true date. I know he has argued in favor of this date for several years, and now he is drawing all the evidence and reasoning together. He bases a considerable portion of his argument on the work of Dr. Taylor Marshall, whose more scholarly article from 2015 is linked at the end of his own. Link
I first found out about Christmas pre dating the Roman feast a few years ago. Despite the evidence, most people still think it’s the other way around. The dating of December 25th is interesting and appears plausible, but even Christians don’t think it was the actual birth date.
How do old calendar orthodox fit in with this? They celebrate Christmas later. Also, how uniform was the date for celebrating Christmas to begin with?