The "Halloween" holiday started as "All Hallows Eve" which was the day before All Saint's Day--a Christian holiday.  This then proceeded to become "All Hallow E'en" which then lead to "Hallowe'en" & eventually our modern "Halloween."  It was the evening before All Hallows Day which eventually turned into All Saints' Day.  ("Hallows" means "saints" in this case).  Pope Gregory IV ordered the church-wide observance of All Saint's Day in 837.  The origin however begins earlier from the common commemorations of martyrs who died in groups or whose names were not known, which were held on various days in different parts of the church.  Over the years these celebrations began to include not only martyrs but all saints.  During the reformation the Protestant churches interpreted "saints" as the New Testament usage which included all believers & thereby reinterpreted the feast of All Saints as a celebration of the unity of the entire church.