Christmas isn't ONLY about Jesus Christ, dude... Christmas is different to every family.
You family may celebrate by talking about God and singing prayers and such like my friend's family does. Your family may also celebrate it by just cooking dinner and then going to sleep. Your family may not even celebrate Christmas at all.
Or your family could be like mine.
To my family, Christmas is no more than a date of socialization. We all get together at my Grandma's house, because her living room is HUGE. We all then sit around, tell stories, watch TV, play games, cook dinner, etc. and then eat dinner and give out presents (cash, little games, stuff like that. nothing expensive), then go home in the wee hours of the morning.
Just because you're an atheist doesn't mean you're condemned from Christmas.

That's like saying if you were an atheist that you couldn't celebrate your birthday.
On what Shreder said: I'm an atheist and I usually get really excited once Christmas rolls around, but it's really not my top priority of the year.
On what Keaton said: I'm an atheist, and I'm not "mooching" off my family to get free things. I don't even care about the presents, I usually get cash/dollar-store brand toys and I don't complain. I could care less.