Why do people who don't believe in Jesus celebrate Christmas?

In many areas people may feel pressured by all the Christmas trees and commercials, and banners and promotions, and sales and other things.

Because Christmas has been made a "national holiday", so technically, anyone can celebrate it.

Christmas has become secularized (the same with Easter). Some of the aspects of the holiday come from paganism, like decorating trees and putting gifts under them. And, people who aren't believers like giving and receiving presents too.