The Dormition of the Theotokos

The Dormition (Falling Asleep) of the Theotokos is one of the Great Feasts of the Orthodox Church, celebrated on August 15. This feast, which is also sometimes called the Assumption, commemorates the death, resurrection and glorification of Christ's mother. It proclaims that Mary has been "assumed" by God into the heavenly kingdom of Christ in the fullness of her spiritual and bodily existence.

According to Orthodox Tradition, Mary died like all humanity, "falling asleep," so to speak, as the name of the feast indicates. She died as all people die, not "voluntarily" as her Son, but by the necessity of her mortal human nature which is indivisibly bound up with the corruption of this world. The feast was added to the Roman calendar in the seventh century as the Dormitio. In the eighth century, the title was changed to the Assumptio (Assumption).
The Apostles were miraculously summoned to this event, and all were present except Thomas when Mary passed from this life. She was then buried.
Thomas arrived a few days later, and desiring to see her one more time, convinced the others to open her tomb. Upon doing so, the Apostles discovered that her body was no longer present. This event is seen as a firstfruits of the resurrection of the faithful that will occur at the Second Coming of Christ. The event is normally called the Dormition, though there are many Orthodox parishes in English-speaking countries with the name Assumption. In Greek, Dormition is Koimisis—falling asleep in death—from which the word cemetery derives.
As with the nativity of the Virgin and the feast of her entrance to the temple, there are no biblical or historical sources for this feast. The Orthodox Church teaches that Mary is without personal sins[1], as well that Mary truly needed to be saved by Christ as all human persons are saved from the trials, sufferings, and death of this world. She truly died and was raised up by her Son as the Mother of Life and participates already in the eternal life of paradise. This life of paradise is prepared and promised to all who "hear the word of God and keep it." (Luke 11:27-28)
The dormition of the mother of the Theotokos, the Righteous Anna, is celebrated on July 25.


Source:

http://orthodoxwiki.org/Dormition