Today is December 21st, 2012, a very special day. No, it's not just the fact that the date is all made up of 1s and 2s (and 0, but let's ignore that). And it's not the fact that it's your last day on Earth either, if the tabloid scientists turn out to be right, anyway, and when have they not been.
It's a very special day since it's the shortest of the year, in the Northern hemisphere, making it the winter solstice there. It's the summer solstice on the other half of the world.
On this day the sun rises to its lowest point in the sky, meaning there's less of it to go around. The effect is greater from higher latitudes, it's going to be a very dark day in Scandinavia for example.
Since June, the days have been becoming shorter and nights longer leading to today, the shortest of the year. Starting tomorrow and for the next six months, days will be progressively longer. If there is a tomorrow, that is.
The solstice has had a significance throughout human history and most civilizations celebrated it in some fashion.
The event is observed or celebrated by plenty of cultures today. Christmas itself, perhaps the best-known celebration in the world, was set to coincide with the winter solstice, which fell on December 25 in Roman times, when Christmas was invented.