In the States (USA) we celebrate Halloween in two ways, and both are excuses to dress up in costumes.
The first way is for children to dress up and go "Trick or Treating". The kids go from door to door and say "Trick or Treat" and get candy, or other edible treats from the people. Originaly the phrase "Trick or Treat" was a threat meaning I will give you a trick if you don't give me a treat. The tricks could sometimes be especially mean such as; throwing toilet paper on trees and houses, or throwing eggs on a house, or soaping windows, etc.
My favorite trick story was one that my Grandfather claimed to have taken part in when he was about 16 years old. A bunch of boys got together and went to a preachers house, dismantled his carriage, and reassembled it on top of the carraige house. When they were finished, one of the people there lit a lantern which illunminated his face (the preacher) and said "That was a lot of fun boys, now lets take it back down".
The second way is for older children or adults to have a costume party and take part in several traditional activities.
At any rate Halloween can be a lot of fun.