The Memorial of the Blessed Virgin Mary, Mother of the Church, was only recently added to the Roman Calendar in 2018, by Pope Francis.  However, Mary has been referred to under the title, "Mother of the Church", going back to at least the 12th century. 

Mary was formally recognized by the Church under this title in a motu proprio issued by Pope St. Paul VI in 1968 (see above). 

"The Blessed Mother of God, the New Eve, Mother of the Church, continues in heaven her maternal role with regard to Christ's members, cooperating with the birth and growth of divine life in the souls of the redeemed."

Pope St. Paul VI
Solemni Hac Liturgia
1968


"The title 'Mother of the Church' thus reflects the deep conviction of the Christian faithful, who see in Mary not only the mother of the person of Christ, but also of the faithful."

Pope St. John Paul II
General Audience
1997