The Yule Lads

The Yule Lads are mischievous Christmas spirits in Icelandic folklore.  The Yule Lads are the sons of Grýla and Leppalúði. They are a group of 13 mischievous pranksters who steal from or harass the population, and all have descriptive names that convey their favorite way of harassing.  They come to town one by one during the last 13 nights before Yule. They leave small gifts in shoes that children have placed on window sills, but if the child has been disobedient, they instead leave a rotten potato in the shoe.

Kallikantzaros

The Kallikantzaroi are malevolent Christmas spirits in folklore.  Stories about the kallikantzaros or its equivalents can typically be found in Greece, Bulgaria, Turkey, Serbia, Albania, Bosnia, and Cyprus. Kallikantzaroi are believed to dwell underground but come to the surface during the twelve days of Christmas, from 25 December to 6 January.  It is believed that kallikantzaroi stay underground, sawing the trunk of the tree that holds the Earth, so that it will collapse, along with the Earth. However, according to folklore, when the final part of the trunk is about to be sawed, Christmas dawns and kallikantzaroi are able to come to the surface. They forget the tree and come to bring trouble to mortals.Finally, on the Epiphany (6 January), the sun starts moving again, and they must return underground to continue their sawing. They see that during their absence the world tree has healed itself, so they must start working all over again.  There is no standard description of the appearance of kallikantzaroi; there are regional variations as to their appearance.