If you don't go to midnight mass on Christmas eve, the trolls will come and steal you away. Or else, you are a troll yourself. This even applies to shepherds who had to keep watch over the sheep (apparently it's impossible for a heathen to become a shepherd, as they're a consistently pious bunch).
I don't know why, but the idea has stuck in my head. If you don't follow the rules of the tribe, the Outlanders will come and get you. Even if the reason you don't follow the rules of the tribe is for the good of the tribe...I just know there's a story in there.