Religions your ancestors believed in contribute to a culture in which you partake, regardless of your feelings on the religion.
Why do you use the word "Thursday?" It is, after all, "Thor's Day," straight from the Norse religion. Are you Norse? No, you're a Christian, but some of your ancestors valued Thor, so it becomes a part of the common culture, passed down to this day.
Similarly, do you celebrate Christmas in December? Because the Bible suggests it was more like May. The December thing is just trying to co-opt pagan practices, including those of the Romans. It's a hold over from *drum roll* your ancestors.
I celebrate Christmas in the same way. It's a heritage thing. Besides, I love a good story.
Christianity needs to have better choices in holidays, though. Birth and death/resurrection of Jesus are the big ones? What about the Sermon on the Mount? Shouldn't that have a big holiday? I mean, if fucking St. Valentine (who didn't, in retrospect the church decided, fucking exist) gets a holiday, how can the most influential Jesus moment not have a holiday?
I vote we get one.