December 21st (approximately)
Yule is one of those sabbats that even non-Pagans know about, though it generally goes by the name of Christmas for most people. Technically, true Yule is a few days before Christmas, but that's not really the point.
The day marks the winter solstice, which is the time of year when the day length is at its shortest. This also means it's the time of the longest night. After Yule, the days will begin to slowly get longer again and that is celebrated as the return of the sun. After a long, dark and cold winter, to finally have more sun again was a huge reason to celebrate.
The underlying meaning of Yule (rebirth of the sun) is the main reason why this was the day chosen by the new Christians to mark the birth of Jesus. That's why so many of our modern-day Christmas traditions don't really have much to do with Jesus, and are filled with old Pagan meaning.
In Celtic myth, this is the day that the Oak King and Holly King do battle. At Yule, the Oak King is the winner and he is in control of nature until Midsummer when they will fight again (which is when the Holly King takes over). Another mythic view is that the Horned God dies at Samhain, and is reborn at Yule.
Other names: Winter solstice, Saturnalia
Traditions and activities: giving gifts, decorating a tree, lighting a Yule log, giving gifts, lighting candles, hanging mistletoe
Correspondences: red, green, silver, cinnamon, pine, myrrh, holly, mistletoe, bells
Foods served: wassail or cider, Yule log cakes, gingerbread, stollen, baked squash, apples