Christmas is an awesome holiday, especially in writing fiction. I Googled it today and found a great post about the history of Christmas - http://www.history.com/topics/christmas - Christmas is a great thing to include in your story, but research is very important if you're doing anything historical. Even if you're not, it's worth looking into. Christmas has gone through quite a bit of evolution through the ages, and if you write fiction, holidays are something that should be considered as part of your world, and it's only understandable that they should continue their evolution. Of course, what evolution the future holds is in your capable hands.
Holidays came to be for a variety of reasons, few of them of a religious nature, at least to begin with. Of course, that depends on your definition of religion too. In fact, some sort of religion has always been around. Worship of the different aspects of nature were likely the earliest samples. This fact is something you should never neglect in your story. It is small background details like this that add life to your story. Even if it's nothing you actually describe, your character's thoughts and actions will reflect some sort of religious teaching, even if it's no more than superstition.
In my book, King by Right of Blood and Might, I took the Christian religion and crushed it, but as we all know, Christianity does not crush well. Through the whole book, I never mentioned Christianity or Christ, or any known form of worship we are familiar with today. I did however keep the statue of the Virgin Mary, but only by description. The worship was more like meditation than worship, and rather than in some kind of church, I went for an outdoor setting not unlike the early worship of the Greek gods with at best a small temple frame, with or without a roof, where offerings could be left. I also connected this worship to something we might call Mother Nature, but as close as I got to that reference was 'the Mother'. I even gave her a presence, a mind, a purpose. I was curious if I would get any comments about all of this, but I'm not sure if anyone actually made those connections, Harris was a much more up-front character.
I'm curious, you that have read my book - Did any of you notice what I did with religion in my book?