the evening before All Saints' Day; often devoted to pranks played by young people
"Kids dressed up and went door-to-door collecting treats on Halloween, enjoying an evening of harmless pranks before All Saints' Day."
The eve of All Hallows' Day; October 31st; celebrated (mostly in the United Kingdom, Canada, United States and Ireland) by children going door-to-door in costume and demanding candy with menaces.
"On Halloween, children dressed as ghosts go door to door shouting "trick or treat" until their buckets are full of sweets."
Halloween is a Scottish shortening of Allhalloweven, which combines "Allhallowmas" (an older name for All Saints' Day) with the word "eve." The term entered English usage as a contraction that merged these two elements to mark the evening before the feast.