On February 11th, 2018, Pope Francis officially decreed that the Memorial of the Blessed Virgin Mary, Mother of the church be inserted into the Roman Calendar to be celebrated each year on the Monday following Pentecost.
The title Mater Ecclesiae title can be traced back to writings of Saint Ambrose of Milan in the early 4th century. The Bishop of Treves, Berengaud, also wrote several letters in which he titled the Blessed Virgin Mary with the same title. As found in the Acts of the Apostles, the Blessed Virgin Mary was seated together with the apostles and disciples at the first Pentecost. In his encyclical Adjutricem populi (Helper of the People), written in 1895, Pope Leo XIII wrote:
"She is invoked as Mother of the Church and the teacher and Queen of the Apostles"
Pope Paul VI, following very similar language to Saint Ambrose (Pope Paul VI was a former archbishop of Milan as was Saint Ambrose) described Mary with her faith, love and unity with Christ as the Model of the Church and Mother of the Church. As the third session of the Second Vatican Council closed on November 21st, 1964, he declared:
"We declare Mary Most Holy Mother of the Church, that is, of all the Christian people"
Saint Pope John Paul II wrote in Redemptoris Mater:
Mary is present in the Church as the Mother of Christ, and at the same time as that Mother whom Christ, in the mystery of the Redemption, gave to humanity in the person of the Apostle John. Thus, in her new motherhood in the Spirit, Mary embraces each and every one in the Church, and embraces each and every one through the Church. In this sense Mary, Mother of the Church, is also the Church's model. Indeed, as Paul VI hopes and asks, the Church must draw "from the Virgin Mother of God the most authentic form of perfect imitation of Christ."
During the UNIV Form in 1980, a young man spoke to Saint John Paul II and informed him that there were 162 statues of saints in Saint Peter's Square but he could not find a single image of the Blessed Virgin Mary. Without hesitation, Saint Pope John Paul II said aloud "Then we should finish the square!". The architect Javier Cotelo was asked to find a solution for the issue and soon submitted a proposal to use one of the windows of the building located between Saint Peter's Square and the Cortile di San Damaso as it could be seen from any place in Saint Peter's Square. On December 7th, 1981, a mosaic of Maria Mater Ecclesiae (Our Lady, Mother of the Church) was installed on the following day, the Feast of the Immaculate Conception, Saint Pope John Paul II blessed the mosaic. It is considered the last stones of Saint Peter's Square as well as a tribute to the intercession of Mary as it overlooks the spot in which an assassin attempted to kill Saint Pope John Paul II.
Pope Benedict XVI referred to the credo of Pope Paul VI in a speech, stating that it sums all of the scriptural texts related to the matter. He wrote that the Church is like Mary - the Church is virgin and mother, immaculate, she suffers and is assumed into Heaven. She learns that Mary is her mirror, that the church is a person in Mary while Mary is not an isolated individual who rests in herself. She carries the mystery of the Church. Pope Francis's decree of the Memorial was signed on February 11th, 2018 which was the 160th anniversary of the Miracles at Lourdes France.