On Halloween everyone in your town will be dressed up like Jack, but there is only one real Jack. Only the real Jack will not chase you. All Jack really wants is candy. If you bought some from Tom Nook's store during the month of October bring it to Jack in exchange for a rare piece of Spooky Series furniture. After you get a piece of furniture from him you can leave that acre and find him again elsewhere in town to get another piece. You can get the whole Spooky Series if you're persistent enough.