atheist we were taught at very high church school that shepherds would not have been out watching their flocks in winter and that the Christian church had taken over an exisiting holiday. Here's the best reason i have ever heard or read from Dr Who.
"Kazran Sardick : On every world, wherever people are, in the deepest part of the winter, at the exact midpoint, everybody stops, and turns, and hugs, as if to say "Well done. Well done, everyone! We're halfway out of the dark." Back on Earth, we called this Christmas, or the Winter Solstice. On this world, the first settlers called it the Crystal Feast."