Elf on the Shelf is a popular tradition that began in the early 20th century. The idea is that a Christmas elf is sent from the North Pole to watch over children and report back to Santa Claus on their behavior. The elf typically arrives on Thanksgiving Day and stays until Christmas Eve, when it returns to the North Pole with Santa Claus. The elf is usually placed in a prominent location in the home, such as a shelf or mantle, to keep an eye on the children and ensure they are behaving properly.