The Winter Solstice is celebrated on the shortest day of the year (i.e. today) and it is traditional to visit your family for a special dinner (no, sorry, that doesn't mean a Big Mac and large fries).  I think we are celebrating the fact that from here on out the days get longer - though compared to the cold and dark December afternoons in the UK I hardly notice the difference.  Anyway, here's the official line:

This celebration can be traced to the Chinese belief in yin and yang, which represent balance and harmony in life. It is believed that the yin qualities of darkness and cold are at their most powerful at this time, but it is also the turning point, giving way to the light and warmth of yang. For this reason, the Dong Zhi Festival is a time for optimism.

The strange thing is that although it is one of the most important festivals of the year, it isn't a public holiday. However, most offices do close early, which is better than nothing.  So, after a short day today we have a full day tomorrow, and then it's Christmas (yes, OK, some people have to work on Saturday morning).  Whoopee.