By decision of Pope Francis, the Roman Rite Catholic Church now celebrates, every year, the memory of the "Blessed Virgin Mary, Mother of the Church" on the Monday after Pentecost.
In 1964, Saint Pope Paul VI gave the Virgin Mary the venerable title of "Mother of the Church" in his speech approving the Dogmatic Constitution on the Church Lumen Gentium. And as Father Claude Touraille explains, "We are far from a mere respectable Marian devotion. We are touching here on the mystery of the Church, and on the fact that the Second Vatican Council dedicated the 8th chapter of Lumen Gentium to Mary, the Mother of Jesus. Her place is at the heart of the Church, and it is a unique place. She is her Mother."
The Pastoral Constitution on the Church in the Modern World, Gaudium et Spes, (§222), teaches that by His incarnation, the Son of God united Himself in some way to every man. It follows naturally that the Blessed Virgin Mary became the Mother of humanity.
Moreover, according to the Council of Ephesus in 431, Mary is Theotokos, that is to say the Mother of God. "Therefore the Son of God, Jesus Christ, is but one person, and He has two natures since He is also truly her Son born of her substance." (Father M.J. Lagrange O.P.)
Saint Augustine, Doctor of the Church, wrote that the body of Jesus is united to the Church, thus forming the total Christ, head and body. This is what we proclaim with this hymn at Pentecost: "We are the body of Christ, each one of us is a member of this body; each one receives the grace of the Spirit for the good of the whole body."
At Pentecost, the Church was instituted, and sent on a mission, under the impulse of the Holy Spirit: "When the work which the Father gave the Son to do on earth was accomplished, the Holy Spirit was sent on the day of Pentecost in order that He might continually sanctify the Church" (Vatican II - Lumen Gentium # 4). By placing the feast of Mary, Mother of the Church, on Pentecost Monday, our Pope wished to remind us that Mary was in the Upper Room with the Apostles on the day of Pentecost and that she has been maternally accompanying the Church in her mission since that day.
Adapted from: Catholic78