Today is the feast of the Dormition of the Most Holy Theotokos, one of the Church’s Great Feasts and the last one of the church calendar year which begins September 14th. The first great feast celebrated in the church year is the Nativity of the Most Holy Theotokos so that the church year begins and ends with our commemoration of the Most Holy Theotokos: her birth at the beginning and falling asleep at the end.
From Bulgakov on the Dormition (here)
“The feast of the Dormition of the Mother of God goes back to the most ancient times of Christianity and in the IV century is already universal, as is evident from the witness of Gregory of Tours and especially that it is mentioned in all the most ancient calendars. In the V century stichera for this feast were written by Anatolius, Patriarch of Constantinople and in the VIII century two canons
attributed to Cosmas of the Holy City and John of Damascus were written. Originally the feast was celebrated on August 18, however that did not prevent its celebration in places on August 15. The common celebration of the feast on August 15 was established by the will of the Emperor Maurice (since the year 582).”