Apparently most Christmas traditions are borrowed from earlier pagan religions. This makes sense, since early humans must have watched anxiously as the winter sun slipped lower and lower toward the horizon. What if it just kept going and never came back? I'm fairly certain the first human inhabitants of, say, Sweden, during their first frigid winter, were certain that was precisely what was happening. Bye-bye sun. Bye-bye life. Looks like we blew it by heading north.
So the joy that resulted from the sun's gradual ascent, after such a worrying dark period, must have been close to ecstasy! (Accompanied, perhaps, by a bit of resentment toward the sun for pulling such a cruel stunt every year!) It also follows then that a celebration would definitely be called for. We're talking party time! Evergreen trees, berries, anything that did not seem to "die" every winter, would be an integral part of any such celebration. Evergreens represent, in a way, everlasting life. So decorate them, dance around them, sing, and party hearty! Enter Christmas.
The overlapping, nearly identical symbolism shared by the rebirth of the sun, and the birth of the "Son," scream out for recognition. Add to that the date of the winter solstice, usually around December 22, and the fact that the most influential god around the Mediterranean when Jesus was supposedly born, Mithra, had his Holy Day on December 25, and "Christmas" is defined. If there was a historical Jesus, no one has a clue when he was born. But it most assuredly was not on December 25.
Moving on then, what's wrong with non-theists co-opting Christmas, just as Christianity co-opted Mithra's birthday? I, for one, am always delighted to see the sun begin its climb in the sky every December. I hate those short, gloomy days when it's already dark long before dinnertime. And while my childhood was unmistakably saturated by my Christian faith, I was also a normal kid. To an eight-year-old looking at a Christmas tree poised over a cornucopia of beautifully wrapped presents, Jesus could come or go—who cared? There was so much fun to be had at Christmas!
Taken from the article An Atheist Christmas by Judith Hayes, a.k.a. The Happy Heretic.