The Vision of the Protection of the Theotokos
Andrew's defining spiritual encounter took place in the Blachernae church of Constantinople while the city was surrounded by enemy troops, identified in some sources as Muslim Arabs. By Sacred Tradition the apparition occurred on Sunday at dawn, October 1, when Andrew saw the dome of the church open and the Virgin Mary enter, moving in the air above him, glowing and surrounded by angels and saints.
The Virgin Mary prayed fervently for all faithful Christians throughout the world, petitioning her Son Jesus Christ to accept the prayers of those seeking her protection, and spread her veil (omophorion) over the people in the church as a sign of divine intercession. Andrew turned to his disciple Epiphanius and asked, in effect, whether he too saw the Holy Theotokos praying for all the world; Epiphanius confirmed the vision with amazement.
After the vision, Constantinople was saved when its attackers retreated. The deliverance attributed to this event inspired the feast of the Protection of the Theotokos.