All Saints Day is celebrated on November 1 by parts of Western Christianity (and on the first Sunday after Pentecost in Eastern Christianity) in honor of all the saints, known and unknown.
According to some sources, the idea for All Saints' Day goes back to the fourth century when the Greek Christians kept a festival on the first Sunday after Pentecost (in late May or early June) in honor of all martyrs and saints. Other sources say that a commemoration of “All Martyrs” began to be celebrated as early as 270 CE but no specific month or date is recorded. Pope Gregory IV made All Saints' Day an authorized holiday in 837 CE. It is speculated that the chosen date for the event, November 1, may have been an attempt to supplant Samhain.
All Saints Day is closely tied with All Souls Day.