Well, for a lot of people---and originally---it's about celebrating the birth of the guy who's the central figure of the Christian religion, though whether or not the guy ever actually existed is up for debate. In the camp of 'yes, there was such a guy' whether or not he was actually divine or just really persuasive or what is also up for debate, but I digress. The amount of emphasis on the religious aspect varies a lot, but even non-religious people---and, sometimes, people who practice other religions---treat it as a time to spend time with their loved ones, exchange gifts, party, and try to be a bit more charitable. Despite my choice in exclamations, I'm not a religious person, but I do like Christmas a lot.