The Dormition of the Theotokos (Greek: Κοίμησις Θεοτόκου, Koímēsis, often anglicized as Kimisis) is a Great Feast of the Eastern Orthodox, Oriental Orthodox and Eastern Catholic Churches which commemorates the "falling asleep" or death of the Theotokos (Mary, the mother of Jesus; literally translated as God-bearer), and her bodily resurrection before being taken up into heaven. It is celebrated on August 15 (August 28, N.S. for those following the Julian Calendar) as the Feast of the Dormition of the Mother of God. The Armenian Apostolic Church celebrates the Dormition not on a fixed date, but on the Sunday nearest August 15.
Read more about Dormition Of The Theotokos: Dormition Fast, Significance of The Feast, Dormition Versus Assumption, Liturgical Practices, Development of The Dormition Tradition