Christmas. Christ's Mass. Founded by the Romans. On a Thursday, probably.
Imagine, for a moment, that you are the burgeoning Roman Catholic Church, keen to assert your authority as (a) the most popular Christian church out there and (b) the most popular church in the Holy Roman Empire. There are two things you need to do. You need to be more official, and you need to be available in an easy-to-digest form that the heathens will swallow.
So you need to fix dates for festivals. And you need those dates to be dates that will be popular with the greasy minions. So you co-opt an old heathen holiday. Saturnalia celebrated the rebirth of Saturn - the god of the harvest, and the dawn of the new year from the winter's darkness. So it's a festival of the solstice, really, co-opted in to the Roman Catholic church to allow the continuation of popular traditions and also, at the same time, cementing the popularity of the Roman Catholic Church. Good old Romans.
This is pretty common knowledge.
Even in the contemporary church, this is one of the least religious of the festivals. The little baby Jesus is simply a baby. He doesn't cry, but he doesn't do any miracles either. He's not a shining example to us, we shouldn't learn anything from his particular piety at his birth. The nearest things to lessons in the Christmas Story are don't go out slaying firstborn sons, presents are good, and if you don't have a spare bedroom, let your guests sleep in the garage. And yes, i appreciate that he's a special baby, but beyond the shiny halo (which must have hurt) he doesn't actually have his superpowers yet.
It's hard to shoehorn religion into Christmas, particularly when it is - and always has been - a festival of overt consumption, self-congratulation and indigestion. It's particularly hard to think of it as a "season of goodwill", when it's a season of last-minute shopping, elbowing grannies out of the way of the last turkey roast and swearing blind that next year you're emigrating to Venezuela.
Perhaps, however, it's as much about fulfilling society's need to vent as it is about fealty to any particular religion. Perhaps the people who rage against the commercialism of Christmas are simply obeying a valid urge to let off steam, and perhaps that commercialism is a response to the needs of society as much as the other way around.
Surely the lesson learnt about first born son slaying was that if you're going to do it... make sure you do it properly.