Many of the customs we follow at Christmas are from the old pagan customs e.g. WTF has Mistletoe got to do with Christianity? Same for Holly and Ivy, greenery in the house, trees.
These customs existed in pre-Roman Britain. As is the way with conquerors, the pre-Christian Romans hijacked the festivals and blended them with their own festivals, eg Saturnalia (Yule) and Quinquatria (Ystre).
When the Roman Empire converted to Christianity (4th century), these festivals became Christian festivals, but retained elements of the pre-Roman practices.
The same happened in other parts of the Roman empire. The Christmas tree has an equivalent in Germanic pagan tradition as does having a ham at Christmas the "Yule boar".
The Romans were excellent at this sort of cultural appropriation. If you visit the Roman baths in Bath, there is a Green Man, a potent pagan image, carved in a prominent position and the Roman name for Bath, Aquae Sulis, honours the pagan diety of the thermal springs, Sulis.
And all this stuff makes me glad I did classics at school. 