How do you get a Halloween costume on Animal Crossing?

First you have to find Jack. He is dressed the same as all of your villagers, but Jack will not approach you when you go near him. Once you have found him, make sure you have all of your candy inside your mail (A very handy way to store extra items in your inventory) by dragging them as presents onto the envelopes. This way when you talk to Jack he will not try and steal your candy. I can't remember if he does steal your candy, Halloween was a while ago. ;) But it's better to be safe than sorry. Anyway, when you speak to Jack he will congratulate you on finding him and ask for candy. Say no and he will punish you by putting a pumpkin on your head. This is just one part of the costume! I have never successfully obtained all of the outfit, but I think you can get more. :D

Sorry for waffling on xD

Hope I helped! :)