From Nookipedia, the Animal Crossing wiki
Naughty-or-Nice Day is an event in the European versions of Animal Crossing: City Folk and Animal Crossing: New Leaf. It occurs on the 6th of December. The player will be notified of this event also if they read the Bulletin Board. Isabelle stands outside the Plaza all day and gives the player a Holiday Stocking, which is exclusive to the event and cannot be ordered from the catalog.