The joke is that they're trick or treating tonight. Halloween is tomorrow.
Living in central Iowa makes me glad I lived where we trick or treated on Halloween and didn't tell jokes (we just said trick or treat and then got our candy).
One of my favorite trick or treat memories was repeatedly going to the town nudists house hoping to get the high school aged daughter or mom to answer the door nude. Unfortunately it never worked but it was still worth a try.
Why does Santa have such a big sack? It was a junior high kid.