Christmas was created to celebrate the birth of Christ. (While it's certainly not his birthday, that is, nonetheless, an accurate and reasonable description.) Those of us who are not Christian but still celebrate Christmas, though, are clearly not celebrating the birth of Christ.
Heck, a lot of people who are only vaguely Christian probably celebrate it for some of the other reasons that LongearedBat mentioned, and not just because it's a celebration of the birth of Christ. Recent surveys in the UK revealed that while a strong majority consider themselves to belong to a religion, a much smaller number consider themselves religious.
You don't have to believe that watery tarts distributing swords constitutes a valid form of government (or whatever it is that the Christians believe--I don't track it all that closely) to believe that all the lights and decorations and exchanging gifts is fun. The Jews may scrupulously avoid celebrating Christmas, but Atheists, Buddhists, and Shintoists, among others, have no such qualms.