YULE -- It is celebrated on the Winter Solstice, around December 21. It is the rebirth of the God as one of the eight Sabbats of the Wheel of the Year. Many of the Christmas traditions come from this holiday, including the hanging of mistletoe and the Yule log.
The Yule log was burned to celebrate the the rebirth of the sun.
Many cultures shared this celebration in different forms like the Egyptians, Celts, Norse (Juul), Romans (Saturnalia), and Germanic tribes.
The burning of the Yule log is a tradition that has been carried on for at least a thousand year or more. A piece from the old Yule log (kept for protection through the year) kindles the new Yule log after a piece is taken from it for the next. There are certain rituals and superstitions about how the log is decorated and received, but I'm not going to get into them.
Disclaimer: None of these pictures belong to me. I found them on Google.