Yule is the next Sabbat. It falls on the Winter Solstice, which falls sometime between Dec 19-22, depending on the year. It is the day when the Earth is at it's greatest tilt, making it the day of the shortest daylight and longest night time. Although the Earth is not actually at the point in it's orbit when it is the farthest from the Sun, Yule is refered to as the time to "bring back the Sun", for it's the sunlight we wish to lengthen.
Yule is not a harvest or planting celebration, but the preparing of the Earth to be fertile again. All Sabbats have the aspect of "turning the wheel of the year", but it's extremely prevalent at Yule. Think how important it was for our ancestors, witnessing the shrinking of the daylight hours, to change this trend and lengthen the day. Given the prevalence of seasonal depression in our society today, it's probably just as important to us modern peoples.
Many of the customs of early Pagans were incorporated into the Christian holiday, Christmas. The giving of gifts, hanging of the holly, kissing under the mistletoe, the tree in the house, and burning of the Yule log are examples. So many of these are prevelant in modern Yule rituals and celebrations. The holly is there to remember the Holly King, the God that rules up till Winter Solstice with a marked resembalence to a guy by the name of Santa Claus. Burning the log shows the Sun how it is to warm us. The evergreen, which never dies in winter, symbolizes rebirth. And the kissing... well... we've always been a bit lusty.
Yule, as all the Solstices, is a time that reflects the cycle of life and rebirth, so in ritual it is reflected. There may be a ritual drama of the battle between the Holly King and the Oak King. In this cycle the Holly King is the old year and father of the Oak King, who replaces him. The Oak King then becomes the father of a new Holly King, who will win back the crown come the Summer Solstice or Litha. Another ritual drama might be the descent of Percephone, to show how winter began, usually emphasizing the descent part. (This is also a Spring theme with the emphasis on the return of Percephone).
So a typical Yule ritual might have chanting to bring back the Sun, along with a ritual drama to symbolize the season and the cycle of life and rebirth, and may end with an exchange of gifts and a feast to celebrate the Sun's return.