When Constantine created the "Universal Church" to unite Europe he took parts of all local religions and convinced everyone (sometimes at the point of a sword) that their religion fit perfectly. The Winter Festival of Nordic religions was the time for rebirth since they were able to detect that the Sun had started north again. It was the first that they were able to notice that the midday shadow was starting to get shorter. Constantine claimed that Christmas was the day that Jesus was born and since nobody was smart enough to contradict, they all went along.