Why is Jesus involved with Christmas, if it was an originally Roman Pagan holiday?
That’s what allowed Christianity to spread so easily into Pagan communities, by “latching on” to existing Pagan celbrations and adapting them into Christian celebrations. It was much easier for people to adopt Christianity if it was already familiar to them. Christmas is celebrated when it is, and with many of the symbols and traditions it has, because it evolved out of the Winter Solstice pagan celebrations with its existing symols and rituals. Similarly, Easter grew out of the fertility celbrations tied to the Spring Eqinox (hence the rabbits and baby chickens associated with Easter).