According to the dictionary:

Halloween (hăl'əwēn`, häl'–), Oct. 31, the eve of All Saints' Day. All Saints' Day is a feast of the Roman Catholic and Anglican churches and a day on which churches glorify God for all God's saints, known and unknown. It is celebrated on Nov. 1 in the West, since Pope Gregory IV ordered its church-wide observance in 837.

..... Click the link for more information. , observed with traditional games and customs. The word comes from medieval England's All Hallows' eve (Old Eng. hallow="saint"). However, many of these customs predate Christianity, going back to Celtic practices associated with Nov. 1, which was Samhain (sä`wĭn), the beginning of winter and the Celtic new year. Witches and other evil spirits were believed to roam the earth on this evening, playing tricks on human beings to mark the season of diminishing sunlight. Bonfires were lit, offerings were made of dainty foods and sweets, and people would disguise themselves as one of the roaming spirits, to avoid demonic persecution. Survivals of these early practices can be found in countries of Celtic influence today, such as the United States where children go from door to door in costumes demanding "trick or treat."