Today, December 21st 2009, is the Winter Solstice / Yule. This is the longest night and shortest day of the year; however, we celebrate that, from now on, the days will (very) slowly begin to last longer.
I want you to imagine for a moment how observant our ancient ancestors were. The time difference between last week, today, and next week is extremely slight; yet, they knew that there was a difference and they calculated the exact date where the change officially occured. That alone is cause for celebration of the human mind (yes, sometimes it can be a good thing!)
Some Yuletide traditions you can partake in: burning a yule log; spiking apples and oranges with cloves (pommander - this is fun and easy and I still have mine from last year!); hanging holly, ivy, and mistletoe around your house and yard; meditating and/or praying thanks to your ancestors, the sun, life, Christmas (if you celebrate Christmas), harmony, humanity, etc.; do spells for peace and love; and work all of the spices, herbs, and colors into your evening.
This very beautiful and detailed article http://www.wicca.com/celtic/akasha/yule.htm explains Yule in depth, and I encourage you to take a look. It will further instruct you as to the history and traditions of this lovely holiday.