Jack is a character in the Animal Crossing series. He is a special visitor who appears in the player's village on October 31. He's a Jack-o-Lantern that gives the player gifts in Animal Crossing.
You can buy candy to give to Jack from Tom Nook's store. On Halloween, you need to find Jack and give him the candy in exchange for a piece of "Spooky" furniture. But it's not that simple. All your neighbours will be dressed up to look exactly like Jack, and if they ask you for candy and you say no, they will steal your candy anyway and turn your head into a pumpkin or change your shirt into a moldy shirt! If you're already wearing a moldy shirt and pumpkin head, they will give you a jack-in-the-box.