All Hallows' Even was shortened to Hallow-e'en by the 16th century. The word Hallowe'en began to lose its apostrophe in the 18th century, though we still have some evidence for the apostrophized version.
Both Halloween and Hallowe'en are dictionary-accepted forms of the day when children dress up and knock on doors asking for candy. As an official holiday, it should always be capitalized, even when it is used as an adjective. The apostrophe spelling is more common outside the United States.
