Firstly it is the last day of the Mayan calendar and the Mayans were a comparatively smart bunch so some people think it probably means that the world will end today. They weren't really that smart though so maybe not. If you want, you could spend today making love with beautiful strangers, buying exotic sports cars on credit or telling your boss what you really think of him - maybe the Mayans were right after all!
It's also the winter solstice, the shortest day, the turning point towards spring and a festival far older than the johnny-come-lately "Christmas", "Hanukkah" and other religious inventions. The Romans were particularly fond of the solstice and came up with "Saturnalia" Originally held on December 17 that celebration became so popular it was extended to December 23rd. That was not surprising since the Saturnalia encouraged excessive indulgence in food, drink and sexual exploits. Schools were closed, citizens wore long hats, and played pranks upon each other in an atmosphere of public revelry. In a reversal of the old social order, masters served their slaves who, in turn, treated their owners with disrespect. Privately, families visited among themselves and exchanged small gifts.
Our calendars haven't always reflected our modern, scientific, understanding of why the sun kept disappearing in winter which is why the secondary festivals mostly take place on the wrong date but, whichever calendar we use, today will be the shortest day