All Saints' Day - n. a Christian festival in honour of the souls in heaven, celebrated on 1 Nov. in Western Churches and on the first Sunday after Pentecost in Eastern Churches. (Followed by All Souls' Day on 1 Nov.)

~Oxford Canadian Dictionary, Edited by Katherine Barber, Oxford University Press, 1998.

The origin of the Festival of All Saint's Day celebrated in the west goes back to 609 or 610 when Pope Boniface IV consecrated the Pantheon at Rome to the Blessed Virgin and all the martyrs.

~ Wikipedia