The thirteenth is the last day of Christmas.
On this night the last of the thirteen Yule lads returns to his parents, the mountain dwelling Trolls Grýla and Leppalúði.
There he is greeted by his brothers and the Yuletide Cat, a beast that eats children that don't receive new clothes in time for Christmas.
According to Icelandic folklore Elves and Hidden people relocate on this magical night.
Seals change their shape, graveyards rise and cows can speak to people.
It is tradition to build a bonfire and have fireworks, but beware of the cows! It is dangerous to hear their message for it will drive you insane.