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：专业英语学习网站 http://www.EnglishCN.com)
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.