In the year 1931 a jubilee marking the fifteenth centenary of the Council of Ephesus was celebrated to the great joy of the whole Catholic world. The fathers at that Council, under the guidance of Pope Celestine, formally condemned the errors of Nestorius and declared as Catholic faith the doctrine that the Blessed Virgin Mary, who gave birth to Jesus, was truly the Mother of God. Prompted by holy zeal, Pope Pius XI determined that the memory of so important an event should continue alive in the Church. Accordingly he ordered the renovation of Rome's famous memorial to the Council of Ephesus, namely, the triumphal arch and transept in the Basilica of St. Mary Major on the Esquiline. His predecessor Pope St. Sixtus III (432-440) had embellished that arch with a beautiful mosaic, but time had done it damage.

In an encyclical Pius XI, moreover, underscored the principal teachings of the General Council at Ephesus, developing in detail and with loving affection the singular privilege of divine Motherhood granted to the Blessed Virgin Mary. He believed that so sublime a mystery should ever become more firmly anchored in the hearts of the faithful. At the same time the Pope singled out Mary, the Mother of God and the one blessed among women together with the holy Family of Nazareth as the foremost model for the dignity and sanctity of chaste married life and for the religious education of youth.

http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2013-10-11