By exalting the divine maternity of the Blessed Virgin Mary, the Church celebrates the most perfect of Mothers, the model for all mothers.
This feast was instituted by Pope Pius XI in 1931, for the 15th centenary of the Council of Ephesus during which the dogma of Mary’s divine maternity was proclaimed.
In the texts of the liturgy, the Church expresses how Mary is the Mother of Jesus, but also our Mother, since it is by her intercession that she obtains for us the grace that unites us supernaturally to her Divine Son. The virginal maternity of Mary applies through Christ, whom she truly engendered in her flesh, to all the members of the Mystical Body of the Son of God.
This teaching was exposed by St. Pius X in his encyclical Ad Diem Illum (1904):
For is not Mary the Mother of Christ? Then she is our Mother also. Therefore all we who are united to Christ, and as the Apostle says are members of His body, of His flesh, and of His bones (Eph. 5:30), have issued from the womb of Mary like a body united to its head. Hence, though in a spiritual and mystical fashion, we are all children of Mary, and she is Mother of us all. Mother, spiritually indeed, but truly Mother of the members of Christ.