The hagiographers for this feast write:
This feast originated in the fourth century at the dedication of a temple in honor of the Mother of God. This church was located between Jerusalem and Bethlehem, which tradition hallowed as the place where Mary and Joseph stopped on their journey to the city of David.
By the fifth century in the Byzantine East, and by the sixth century in the Roman Church, this day celebrated the death of Mary, her dormition or “falling asleep” as it is called in liturgical poetry.
Apocryphal accounts, iconography, and texts of the feast are embellished with a persistent pious tradition that all the apostles returned to her deathbed from their missionary journeys.
Through the feast of the Dormition, the Church regards Mary as the first to participate in the final deification of all creation. This is only fitting for the Mother of Life, through whom God became one of us, to die and, by his rising, make the passage from death to life an eternal reality. (NS)