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.
Most know that Hallowe’en is a contraction of All Hallows’ Evening, but what you may not know is that the day originated with the Celtic calendar, which marked the first day of the year as November 1st. When the Catholic Church named November 1st All Saints’ Day or All Hallows, October 31st became All Hallows’ Eve, like Christmas Eve. And like Christmas, All Saints’ Day can be called Hallowmas.