The feast day celebrates the appearance of the Mother of God at Blachernae (Vlaherna) in the tenth century. At the end of St. Andrei (Andrew of Constantinople) Yurodivyi’s life, he, with his disciple St. Epiphanius, and a group of people, saw the Mother of God, St. John the Baptist, and several other saints and angels during a vigil in the Church of Blachernae, nearby the city gates. The Blachernae Palace church was where several of her relics were kept. The relics were her robe, veil, and part of her belt that had been transferred from Palestine during the fifth century.

The Theotokos approached the center of the church, knelt down and remained in prayer for a long time. Her face was drowned in tears. Then she took her veil (cerement) off and spread it over the people as a sign of protection. During the time, the people in the city were threatened by a barbarian invasion. After the appearance of the Mother of God, the danger was averted and the city was spared from bloodshed and suffering.

Father pointed out yesterday that ironically, though this is a favorite feast among the Slavic people, the barbarians at the gates were actually Slavs!

Another tidbit:

In recent years, the Feast of the Protection has become associated with thanksgiving for the deliverance of the Greek nation from the Italian invasion of 1940. These events are commemorated in Greece in a national holiday known as “Ochi Day” or “No Day,” referring to the response of the Greek leader Metaxas to Mussolini’s ultimatum.

In recognition of this, the Holy Synod of the Church of Greece elected in 1960 to transfer the Feast from October 1 to October 28. The Ecumenical Patriarchate also provides for this usage in its parishes in Greece and in the Greek diaspora, and it is generally observed now throughout the Greek-speaking world. The observance includes the chanting of a Doxology incorporating hymns recognizing the Protection of the Theotokos over the Greek nation, as well as the kontakion “O Champion Leader.”

–Orthodoxwiki