Why do atheists celebrate Christmas?

Simple answer:  Not all of us do.

I have friends that only celebrate the solstice. These are the atheists that keep with the tradition of how the holiday celebration started- by pagans rejoicing in the movement of the sun.

I have friends that put up a Festivus pole. Some place a Flying Spaghetti Monster on top of their tree. These examples are of the commercialism that has dominated the holiday for decades.

I have known some atheists who have Jewish heritage that will still celebrate Jewish holidays. It is a part of their culture.

Many go through the religious motions of Christmas because of pressure from society or their family.

Personally, Christmas has always been about my family traditions. My family has dozens of small things we do to celebrate the season. None of it really has anything to do with a baby in a manager.

Even though my family is religious, I grew up with my mother telling me that Jesus was not born in the winter months. I'm sure this thinking was not taught to her in Sunday School. I'm not sure where she was able to learn this knowledge but, nonetheless, she always taught it to me. (And she wonders how I ended up a godless heathen)

So, for me, Christmas as always been about presents, PJs on Christmas Eve, Santa Clause, breakfast burritos, story telling, leaving carrots for Rudolph, opening 'sister presents' on Christmas Eve and my mother reading Twas the Night Before Christmas.

Why do atheists celebrate Christmas? Because it's not and never was about your particular messiah.