Scientology is not a spin off of Christianity so Christmas is not a special day as such in the religion. However, many Scientologists are Christians or were raised in the Christian tradition so they celebrate it.
One of the great things about Scientology is the fact that you can be a Scientologist and a member of any other religion. Scientology is not an "exclusive" religion like the traditional western religions - In the west you are a member of one religion and no others.
Scientology follows the tradition of some eastern religions where you can be a member of as many religions as you like because the key thing in Scientology is wisdom. Scientology deals with things that an individual can "take or leave" because it is practical. You can try it out and see if it works or not. So, for example, you can try out the Study Technology that was developed in Scientology and see for yourself if it works or not. Meanwhile, you can believe in God, or not as the case may be, because Scientology doesn't have any dogma on that subject. It leaves it up to the individual.
So you can be a Scientologist (someone who uses Scientology in their life to improve their life and the lives of others) and you can also be a Christian and worship God in the Christian manner.
It's a new concept for the west, but a good one, because it promotes tolerance and harmony and isn't that what Christmas is all about?