It's the shortest amount of daylight (for those of us in the Northern Hemisphere, anyway, those in Oz are having their longest day of the year).
Just think, not even a couple of thousand years ago, this day fell on Dec. 25th. This was at the time when the Christians were trying to push out all of Paganism. They couldn't quite do it. So, they turned the Pagan Holiday of Winter Solstice into a Christian Holiday of Christmas, spouting some nonsense that Jesus was born on that day (they had to make up something, didn't they?). Fortunately, because of the Earth's wobble, Winter Solstice dates move forward and backward, even if not noticibly perceptable in one man's lifetime... I mean, it took this long just to move 4 days! At one time, Winter Solstice actually occured on January 1st, which is why we still celebrate that date as the beginning of a New Year. To give you an idea of how long Paganism was around versus Christianity, just think about this. In the time that Christianity has been around, the Solstice has only moved 4 days (from Dec. 25th to today's Dec. 21st). On the other hand, when the Pagans began to keep track of these kinds of things, they celebrated the solstice on January 1st. By the time the Christians tried to rid the world of the pagans, the solstice had moved 7 days (from Jan 1st to Dec. 25th). The pagans had been around at least double the amount of time that the Christians have. -- Just an interesting side-note, if you were interested.
So, today is the first day of Winter. One of four seasons which we humans have split the year into. It will last for three months, or 'til March 21st, whereby Spring will begin, as noted by an Equinox, or the equal amount of day and night, due to the sun's light shining directly over the equator. Summer Solstice will then occur 3 months after that, then the Vernal (or Autumn/Fall) Equinox, and finally another Winter Solstice, all separated by three months.
Happy Winter Solstice!