Halloween is a strange holiday. It started several centuries ago in Europe. It began as "Hallow Evening" meaning "holy night" At that time November 1 was called "All Saints Day" and the people honored all the saints that did not have their own special day. The night before that, October 31, became a time when all the evil spirits were supposed to be active. (来源：EnglishCN英语博客基地)
On "Hallow 'een", all the bad spirits came out to do mischief.
People would play all sorts of pranks on each other. Practical jokes, Put a cow in the neighbor's house, things like that.
Now it has evolved to a children's holiday. Small children go "Trick or Treating" They wear costumes, knock on doors, and yell, "Trick or Treat". People give them candy and treats.
The original idea was a form of friendly extortion. "Give me a treat, or I'll play a trick on you." The threat was, something like, "I'll spill all the trash out of your trash can if you don't give me a treat."
It's a lot of fun for children. They dress up like ghosts, witches, and such.
Adults sometimes "trick or treat" their friends too. Usually that means the "treat" will be a glass of wine or something.
That is it, Halloween, the 31 of October, a holiday for fun.