The earliest traditions of the Church in the East celebrated the Dormition of Mary or the "Falling Asleep" of the Virgin. Mary died, but because of her preservation from sin as the Mother of God, she entered eternity completely, body and soul. Indeed, the house where she was said to have died in the company of the remaining apostles, existed into the fourth century. The Western Church celebrates this holy event as The Assumption of Mary, August, 15.
The Assumption was based upon the biblical words of the angel Gabriel at the Annunciation about Mary's holiness (Luke 1.28), her continuing faithful witness at the Cross, and later symbols that imagined her as the Church in the book of Revelation: "A great sign appeared in heaven, a woman clothed with the sun, the moon beneath her feet, and a crown of twelve stars on her head." (Revelation 12.1) Mary, as the Mother of God, as the Mother of the Church and all faithful believers, was to triumph over sin and death, through the power of her son Jesus.
During the middle ages, Christian piety proclaimed this beliefin the fourth glorious mystery of the Holy Rosary. But it was only in 1950 that Pope Pius XII, after consultation with the bishops, defined the Assumption as a dogma of the Catholic Church: "With the course of earthly life brought to completion, the Immaculate Mother of God, the ever Virgin Mary, was taken body and soul into celestial glory." And according to the Catechism of the Catholic Church, "The Assumption of the Blessed Virgin is a singular participation in her Son's Resurrection and an anticipation of the resurrection of other Christians." (par. 966)