Although I don't believe in God or that Jesus was anything other than a man, I still celebrate Christmas and Easter in my own way. Why?
1) It is a national holiday that is part of our culture like the 4th of July. Look, I am not Catholic, but I still celebrate St. Patrick's Day and St. Valentine's Day. I don't believe in ghosts and goblins, but I still partake in the Halloween activities.
2) All of my extended family still celebrate Christmas. It is a family thing. It is fun to join in all of the festivities and traditions, including giving and getting gifts, and visiting each other. We even still put up our own lights and tree.
3) Many of the Christmas traditions aren't Christian anyway, but have pagan roots, such as Christmas trees, 25th of December, etc. And Santa is far from any religious message as well.
Now, as my son grows, we will be sure to tell him how we do not believe Jesus was a Savior or that Santa or the Easter bunny are real, but that does not stop us from enjoying this traditional, national holiday.