In the time of Emperor Leo the Great (457-474) and Empress Verina and Patriarch Gennadius, two noblemen from Constantinople, Galibus and Candidus, were traveling in the Holy Land to venerate the holy places. They stopped in Nazareth for a while, at the home of a Jewish maiden who kept the robe of the All-holy Mother of God in a secret room. Many who were ill and in need received healing of their sufferings through prayer and touching the robe. Galibus and Candidus brought this holy object to Constantinople and reported it to the emperor and patriarch. This brought about great joy in the imperial city. The robe was solemnly placed in the Church of Blachernae. (This church had been built by Emperor Marcian and Empress Pulcheria on the shore of an inlet, and was called Blachernae after Blacheran, a commander from Scetis, who had been slain there.) This feast was instituted in remembrance of the placing of the robe of the All-holy Theotokos in the Church of Blachernae.
From The Prologue of Ohrid by St. Nikolai Velimirovic
O godly shelter that dost cover all mankind, the sacred robe that covered thy sacred body hast thou bestowed on all the faithful graciously, O pure Virgin, as a robe of divine incorruption. As we celebrate with love its august deposition, we cry to thee with fear, O graced of God: Rejoice, O modest one, boast of the Christian race.