Today marks the arrival of the first of the Yule-lads (IS jólasveinarnir); mischievous spirits who arrive one per day for the next twelve days, and each stays for exactly thirteen days, so on the 25th of December, they're all present. They are the sons of the Icelandic trolls Grýla and Leppalúði, and each has a specific attribute.
Of course, in modern times their hard edges have been smoothed over, and they're seen as mostly-benevolent, Santa-like figures, but all the good wishes in the world won't change the fundamental nature of a wight. They're basically cautionary tales for children, as they would come out of the mountains and glaciers to frighten naughty children during the Yuletide.
Now that I've seen this story about a woman in Peoria who has the Yule Lads on her lawn as decorations (much like Christians might have a nativity), I want a set of my own for next year!
So today, watch out for Stekkjarstaur, and keep your sheep safe.