Halloween is a holiday in many English speaking countries that is celebrated on the night of October 31st.
Children wear costumes and they go to people's homes saying "Trick or treat!" to ask for candy and then people give it to them. This comes from a threat. It means, "Give me a treat or I will play a trick on you."
Children today usually do not do the tricks if they do not get treats. But some children still do mischief (pranks or things to make fun of people like putting toilet paper in trees or writing with soap on windows or throwing eggs at their house). People sometimes dress up as ghosts, witches, goblins, and other scary things for Halloween. (Source:Simple English Wikipedia)