It's not just being a Christian that makes a person celebrate Christmas. The whole symbology of birth and renewal is just a basic human response to the changing of the seasons, the winter solstice, and the like. When we are at twilight, we dream of the dawn. When we face death, we dream of birth. It's just how we defy our surroundings in order to survive.
That's probably how the original pagan idea of Saturnalia formed from an idea into a ritual, then over time a tradition, then eventually was reformed into Christmas by the religious machinations of empires.
Of course, if it really is all about seasons and solstices, that DOES mean that those in the Southern Hemisphere could celebrate at a different time of year, but that would mess with dating systems and consumerism (i.e. when people from the Northern Hemisphere sell their calendars southwards and the like) if it ever really took off. But it does seem fair that we can't say that atheists should celebrate it "due to the solstices", particularly if they live in the South of the world.
Instead, I think celebration for its own sake is a good enough reason. Eating, drinking and being merry is how humanity forms communities. Whatever the excuse is to down an alcoholic beverage, people will do it anyway. That's how we come together. Tradition came before religion, and it stays after religion too. Which is certainly not always a bad thing.
