When Zippy returned to the North Pole with Santa he left a special present for the kids, a new Elf on the Shelf book!
This one explains how, if the kids write to Santa, Zippy will be allowed to return for 24 hours for their birthday. He will decorate a chair like a tree (cause that's what elves do) and wear a special cupcake outfit. So after dinner Big Brother "wrote" his letter to Santa asking Zippy to come for his Birthday after he decided that he wanted Zippy here for his actual birthday rather than the day of his birthday party. We'll have to see what Zippy has done when we get up in the morning.