All Hallows' EVE, commonly known as Halloween, is an annual holiday celebrated on October 31st. It is believed to have originated from the ancient Celtic festival of Samhain, which marked the end of the harvest season and the beginning of winter. The term 'All Hallows' EVE' is derived from the phrase 'All Hallows' Even, ' which means 'evening before All Hallows' Day.' The answer to the crossword puzzle is EVE, which is a shortened form of the phrase.