Apparently , they moved his birthday to December to ' smother ' pagan celebrations of Saturnalia and the winter solstice , filching many of the associated customs for good measure.
https://www.history.com/topics/ancient-rome/saturnalia#:~:text=Saturnalia%2C held in mid-December,%2C feasting and gift-giving.
Just as the first churches in Britain took over pagan sites of worship.
It was the same with Islam. Mohamed took all the pagan customs and rolled them all into his new religion. Great salesman he was. They bought it.