The Memorial of the Blessed Virgin Mary, Mother of the Church is celebrated on the Monday following Pentecost, highlighting the fact that the Mother of God was praying alongside her spiritual sons, the Twelve Apostles, at the moment when the Holy Spirit descended upon them at Pentecost. Mary, by virtue of being the mother of Jesus Christ, has always been recognized as the Mother of the Church.
Since we regard Pentecost as the birthday of the Church, it is fitting to celebrate this next day as the feast of Mary, Mother of the Church. She was present, abiding in prayer with the Apostles in the Upper Room as they received the Holy Spirit, and it was in fact her “yes” to following God’s will that set in motion our salvation as she became Jesus’ mother, his first and greatest disciple, and our first and greatest example in faith.
The Church today continues to encourage all of us to entrust our lives, our vocation, our ministry, our service to Mary, the mother of Jesus, mother of God, mother of the Church.
(Excerpts from catholic.mylife and catholicculture)