The Vladimir icon
The Feast of Mary Mother of God was instituted by Pope Pius XI in 1931 in view of fifteenth centenary of the Council of Ephesus. Oct.11, 431 AD was the date historians believed to have been the date of the close of the the Third Ecumenical Council. In the Council of Ephesus Mary was proclaimed Theotokos the Mother of God.
Mary is devoutly honored as Theotokos in the Eastern Church, and the pope, in instituting the feast, asked prayers for Christian unity.The feast day was moved by Pope Paul VI to January 1, Solemnity of the Mother of God.