because, contrary to what pagan larpers say, christians weren't anti-pagan traditions. they in fact encouraged the germanics to keep their traditions as long as they didn't involve worshiping false gods or doing evil like child sacrifice or debauchery.   there's proof that christians have been celebrating the birth of christ on december 25th dating back to the second-third century. it has to do with the nine month period between christ's birth and crucifixion.
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