Kallikantzaros are a sort of Christmas Goblin or bogeyman; children born on Christmas Eve or during the 12 days of Christmas. These days were set apart from the rest of the calendar as both blessed and dangerous, and certainly more open to travel between worlds. During this period the Kallikantzaros were allowed to plague humans, causing mischief, and potentially harming people and animals. In language, the name is also used for vampires and werewolves. One way to prevent a child born at this time was to burn or singe his toenails, and some accounts have this done by putting the baby’s feet in the fire, not trimming the nails and tossing those in. (They must have been VERY afraid of this fate!)
Boxing Day, the day after Christmas there used to be a custom called Hunting the Wren, in which boys would go out and kill a small bird, put it on a bier of green branches and carry it house-to-house begging for money for a funeral for the bird. This is pretty weird.