I find the myriad of opinions on the religious threads to be mind-boggling. In any case, given the weather in Northern Europe (and my own area of the northern US), I can't think of a better time to celebrate the birth of the Saviour: when the world turns dark and depressing and frightening, when the cold seeps into your bones, that's when humanity feels an innate need for light and to worship the Light of the World: Jesus Christ.
That's generally what Christians do every week, not just once a year.