on halloween eve, children would go door to door with pumpkin lights, dressed in all sorts of bizarre costumes, asking for candy. constantly saying, "trick or treat" (which means, "give candys to me or i'll make a mess.") if you refuse to give candy, the kids get angry and punish you in various ways, such as dumping trash in your house, etc. they will make tricks on you until you're willing to give them candy.
traditionally, people dress like elves and go from house to house begging for food. people believe that the elves should be worshipped and fed to be pleased, otherwise the elves will play tricks, such as blocking chimneys,hiding cattles, etc. other people who would like to fool or frighten away these evil spirits, will be dressed up as them, so that they believe the evil spirits will not harm them.