Dormition of Theotokos
The Dormition of the Theotokos is the Eastern Orthodox commemoration of the "falling asleep" or death of Mary, the mother of Jesus. It is celebrated on August 15 as the Feast of the Dormition of the Mother of God. It is preceded by a two-week fast from meat, dairy and oil. It is similar in subject matter, but ontologically different from, the Roman Catholic feast of the Assumption of Mary.
In Orthodoxy, death is often called one's "falling asleep," a prime example being the name of this feast. (There is also the Dormition of Anna, Mary's mother.) The Orthodox believe that Mary, after spending her life after Pentecost supporting and serving the nascent Church, became ill. The apostles, scattered throughout the world, are said to have been miraculously transported to be at her side when she died. The sole exception was Thomas, who was characteristically late. He is said to have arrived three days after her death, and asked to see her grave so that he could bid her goodbye. When they arrived, her body was gone, leaving a sweet fragrance. An apparition is said to have confirmed that Christ had taken her body to heaven after her soul and reunited them, as a foretaste of the general resurrection to come.
Dormition versus Assumption
The Dormition of the Theotokos is celebrated on August 15, the same calendar day as the Roman Catholic Feast of the Assumption of Mary. The Dormition should not be confused with the Assumption. Eastern Orthodox belief is outlined above, whereas Roman Catholic dogma requires a Roman Catholic to believe Mary was assumed into heaven, with or without having died first.
Eastern Rite Catholic observance tends to fall somewhere in the middle, but many Eastern Catholics celebrate the Dormition after a fashion, in keeping with both Catholic dogma and their Eastern predilection.