This feast, celebrated on August 15 following a fourteen day fast, is the last Great Feast of the Orthodox liturgical year. I find it interesting and fitting that their liturgical calendar begins and ends with a feast of Mary. Dormition means ‘falling asleep’ using the Christian term from the New Testament for death. The term reflects our belief that death has been defeated by Christ; the metaphorical gates of Hades or Sheol have been burst asunder and death no longer enslaves humanity.
Tradition holds that the apostles were miraculously summoned and, except for Thomas, were all present when Mary reposed. Thomas arrived a few days later and desiring to see her one more time, convinced them to open the tomb. When the tomb was opened, it was found empty. This event is seen as one of the firstfruits of the resurrection of the faithful.
The feast is celebrated as the Solemnity of the Assumption by the Roman Catholic Church and focuses on her bodily assumption rather than her death. In fact, the dogma is phrased in a way that leaves open the question of whether or not Mary experienced death at all and many Catholics believe she did not. Pope Pious XII made the Assumption a dogma of the Catholic Church on November 1, 1950 as follows.
By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
As with other such dogmas established in Catholicism as acts of Papal Infallibity, the Orthodox perceive this as another addition to the faith by the Catholic Church, widening the schism between the two. In this case, unlike the dogma of the Immaculate Conception, the Orthodox essentially agree on the event itself. But the Orthodox believe it is preserved in the faith through the liturgical life of the Church and not as a dogma.
Below is a recording of an ancient hymn of the feast in English.