I love Halloween, I think that traditions originated from ancient Celtic harvest festivals
Cool. When did trick or treat become a tradition during Halloween?
I do not know, but I love  trick-or-treating, attending Halloween costume parties, carving pumpkins into jack-o'-lanterns
I like all that stuff too. What's your favorite halloween costume?
I love superheros costume, Halloween is dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.
Do most countries celebrate Halloween? 
No actually, some people believe, that Halloween began solely as a Christian holiday, but I don't think so.