All Saints' Day, feast of the Roman Catholic and Anglican churches, and 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. Its origin lies earlier in the common commemorations of martyrs who died in groups or whose names were unknown, which were held on various days in different parts of the Church; over time these celebrations came to include not only the martyrs but all saints. During the Reformation the Protestant churches understood "saints" in its New Testament usage as including all believers and reinterpreted the feast of All Saints as a celebration of the unity of the entire Church. In medieval England the festival was known as All Hallows, hence the name Halloween [ = All Hallows' eve ] for the preceding evening.