The first mention of the custom of selecting and burning a giant log at Christmastime was in 1184, but it’s likely much older than that. In Europe in the Middle Ages, Christmas celebrations would generally last for 12 days.
When the feast was over, the log would be kept in the house because of its magical ability to ward off a wide variety of diseases and ailments, and the following Christmas, it would be used to light that year’s yule log.
No one is quite sure where the tradition got started, but at least some scholars believe that it might be a watered-down adaptation of the ancient Celtic custom of human sacrifice.