Established in Poland in 1971.
Mary is the Mother of the Church. Theologians have given the Mother of God this title from the beginning of the Church, as in the past century the popes Leo XIII, John XXIII and Paul VI have done. The Polish bishops submitted a Memorial [??] to Paul VI, with the fervent plea that he declare Mary the Mother of the Church, and once more give the whole family of mankind to her maternal heart. On 16th September 1964 during the third session of the Vatican council, Stefan Cardinal Wyszyński, in the name of seventy Polish bishops, gave a speech [there's a word for this, isn't there? Anyone?] in which he showed the necessity of declaring Mary the Mother of the Church. He drew on the experience of the Polish nation, for whom the Mother of Christ, present in its history and whom it had always invoked, had been refuge, help and victory. The Polish bishops also worked hard to see that doctrine about Our Lady was included in the Constitution on the CHurch, since this underlines the dignity of Mary as the Mother of the Church, and her active presence in the mystery of Christ and the Church.
In 1968 Paul VI confirmed his declaration on the Mother of the Church in the “Pauline Creed”. The Polish episcopate then added a new invocation to the Litany of Loretto, “Mother of the Church, pray for us”. They also appealed to the Holy See for this petition to be added to the Litany throughout the Church, and that the feast of Our Lady Mother of the Church be likewise established in the whole Church.
[from brewiarz.pl, trans. bat Ionah]