Phineas and Ferb's Dr. Doofenshmirtz gives us his take
Unlike the bad doctor, I in fact love Christmas. So what? Millions of people love Christmas. It is true, but most of them are religious to one degree or another. I on the other hand, am a committed atheist. I do not believe in God, or gods, or soul, or spirit, or astrology, or the power of crystals. Most atheists hate Christmas, and at this time of year, you will find them expounding on various aspects of the holiday that disturb them, from the fact that it is an amalgam of various pagan holidays, to the fact that it's not really the date of Jesus's birth, to the fact that some Christians choose this opportunity to become aggressive about their beliefs, or to the fact that the U.S. is supposed to be a pluralistic and secular culture. Again, so what? Just because all of these things are true, is no reason to chuck one of the funnest set of rituals we have.
Full disclosure, I was not raised in a religious household, though both of my parents came from a protestant background, and my mother clearly retained those beliefs. We did not attend church as a family, but my mom read us Bible stories and told us they were true. We celebrated all of the major Christian holidays with gusto, and most of my relatives are practicing Christians of the low church, protestant variety. We were taught that Santa Claus was real, and I believed in Rudolph, Frosty the Snowman, toy-making elves, and practically anything else you can think of. And I thought it was magical and wonderful! I was not in any way traumatized when I discovered that these things were imaginary. In fact, I continued to pretend to believe for a number of years after I stopped actually believing. I can remember lying in bed at night, getting myself all psyched up for Santa's visit, and being delighted at the full stockings and presents under the tree on Christmas morning, despite my complete understanding that my mom did all of the work. It was still transformative--a beautiful day of light and plenty at the darkest time of the year. And that was only the beginning! Later in the day, the house would fill with family and feasting. There would be tons of laughter and special treats and game time after dinner, with multiple generations sharing in the fun.
So my particular psychological circumstances lead me to continue to embrace Christmas, but I also have cultural grounds. I love tradition, and I don't see the need to reject a harmless practice, based on a lack of religious belief. I'm pretty sure nobody still believes that one's soul flies out the mouth during a sneeze, and yet countless people continue to say "Bless you" as a means of expressing concern for the health of our loved ones. One of the best ways to look at celebrations is to conduct a thought experiment. Imagine yourself visiting a foreign culture, and finding yourself in the midst of joyous holiday preparations. Everything is completely unfamiliar to you, but your hosts graciously invite you to join in the festivities, promising delicious foods, singing, decorations, games, drinks, and a generally raucous good time. You are welcomed as a guest and treated as an esteemed friend. I cannot believe that there are many atheists who would turn down such an opportunity to celebrate Diwali, say, in India, yet they resentfully refuse to participate in their own culture's festive time. If you truly reject religion, then you are free to embrace the secular aspects of Christmas, and the fact that so many pagan traditions are woven into our celebration makes it all the more fun and interesting to me.
So Jesus wasn't even born on December 25? And? Maybe he never even existed at all as a historical personage. The non-literal nature of myth does not lessen its power. We don't cease to study the powerful stories of the ancients, because we doubt Zeus's supremacy. Jesus wasn't truly the son of God, but the story of His divine birth is arguably the very best part of Christianity. It is the story of the mundane miracle of human life. Every birth is remarkable, even those that take place in the most humble of circumstances. Every human being, pauper or king, comes into the world in the same way. We begin our lives as equals, and we are all in possession of a little bit of the eternal, the transcendent, simply by taking part in the beauty and the terror that is conscious existence.
Christmas is the way generations of people have decided to take a stand against the cyclical darkness and chill. So we blaze up our windows and lawns, we grab a little greenery and bring it inside, we toss in anything with color or shimmer, we celebrate the birth of a baby when confronted with the death of our trees and crops, we feast while our stores are high in preparation for the lean months ahead, we sing into the silence, we announce our presence and we assert our will before the overwhelming power of nature.
N.B. Even though it should go without saying, I will still say that we need to always respect the stories and traditions of others (including practicing Christians in our own nation) and shouldn't make assumptions about people's beliefs or lack thereof. Offering a hearty "Season's Greetings!" should cover all the bases without offense, and graciously accepting a "Merry Christmas", "Happy Hanukkah", "Blessed Kwanzaa" or "Happy Diwali" should be the obvious response when wished well by a practitioner of another faith.