All Saints' Day, often shortened to All Saints, is a solemnity celebrated on 1 November in Western Christianity, and on the first Sunday after Pentecost in Eastern Christianity, in honour of all the saints, known and unknown. In the Roman Catholic Church ist is officially called the Solemnity of All Saints and also called All Hallows or Hallowmas.
In terms of Western Christian theology, the day commemorates all those who have attained the beatific vision in Heaven. It is a national holiday in many historically Catholic countries. In the Roman Catholic Church, the next day, All Souls' Day, specifically commemorates the departed faithful who have not yet been purified and reached heaven.
Christian denominations celebrate the Solemnity of All Saints' Day and the Feast of All Souls' Day because of the fundamental belief that there is a prayerful spiritual communion between those in the state of grace who have died and are either being purified in purgatory or are in heaven, and the 'church militant' who are the living. Those who have died and are with God watch over those still living, and the saints are held to intercede with God on behalf of the living. On their part, the living pray to the saints and remember in intercessory prayers to God all who have died, particularly their deceased relatives and friends. (From: Wikipedia)