It's Winter Solstice, the day in the Northern Hemisphere when the Sun moves into the first degree of the sign of Capricorn. The shortest day and the longest night. It's the celebration of the (re)birth of the Sun, who nine months ago was in the sign of Aries, the sign associated with beginnings and the start of anything.
Capricorn is ruled by Saturn, a much-misunderstood planet, that gets a bad name at times for the stern and serious approach to life it seems to favour. But it's no coincidence of course that the figure of Santa Claus, who only makes an appearance at this time of year, embodies the spirit of Saturn and Capricorn. The old man of the skies is an important figure to acknowledge in order to really get the meaning of this part of the zodiac, as he's a reminder that Saturn isn't really about harsh lessons, and Capricorn does more than just stick to the rules.
Peace and goodwill to all, through fun and the enjoyment of giving, is the real Solstice message, something that we all need to work hard at the whole year round, to reap the real rewards that Saturn has to offer.
Mike Nichols has a great page on the history and traditions of Yule here, and there's an indepth look at Capricorn, with masses of info on Saturn too, at Deborah Houlding's site here.