The Protection of the Mother of God is one of the most beloved feast days on the Orthodox calendar among the Slavic peoples, commemorated on October 1. The feast is celebrated additionally on October 28 in the Greek tradition. It is also known as the feast of the Virgin Mary's Cerement.

In most Slavic languages the word "cerement" has a dual meaning of "veil" and "protection." The Russian word Pokrov (Покров), like the Greek Skepi (Σκέπη), has a complex meaning. First of all, it refers to a cloak or shroud, but it also means protection or intercession. For this reason, the name of the feast is variously translated as the Veil of Our Lady, the Protecting Veil of the Theotokos, the Protection of the Theotokos, or the Intercession of the Theotokos

According to Eastern Orthodox Sacred Tradition, the apparition of Mary the Theotokos occurred during the 10th century at the Blachernae church in Constantinople (modern-day Istanbul) where several of her relics (her robe, veil, and part of her belt) were kept. On Sunday, October 1 at four in the morning, St. Andrew the Blessed Fool-for-Christ, who was a Slav by birth, 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. She knelt and prayed with tears for all faithful Christians in the world. The Virgin Mary asked her son, Jesus Christ, to accept the prayers of all the people entreating him and looking for her protection. Once her prayer was completed, she walked to the altar and continued to pray. Afterwards, she spread her veil over all the people in the church as a protection.

St Andrew turned to his disciple, St. Epiphanius, who was standing near him, and asked, "Do you see, brother, the Holy Theotokos, praying for all the world?" Epiphanius answered, "Yes, Holy Father, I see it and am amazed!"

An icon of the Virgin Mary praying, surrounded by people, was said to be kept in the Blachernae church. It is said to reproduce the events as St Andrew saw them that day. (source: OrthodoxWiki)