Motherhood Of The Blessed Virgin Mary

To commemorate in the liturgy the fifteenth centenary of the Council of Ephesus (held in A.D. 431), which vindicated the title of Theotokos or "Mother of God" for our Lady, Pope Pius XI in the year 1931 instituted this Feast to be observed by the whole Church.
The Roman Missal, 1962

Introit (Isaias 7.14)
Behold a Virgin shall conceive, and shall bear a Son, and His name shall be called Emmanuel. Ps. 47.1. O sing unto the Lord a new song, for He hath done marvelous things. Glory be to the Father, and to the Son and to the Holy Ghost. Behold a Virgin shall conceive, and shall bear a Son, and his name shall be called Emmanuel.

Collect
O God, who was pleased that, at the message of an Angel, Thy Word should take flesh in the womb of the blessed Virgin Mary, grant to us Thy suppliants, that we, who believe her to be truly the Mother of God, may be helped by her intercession with Thee. Through the same Jesus Christ our Lord, who lives and reigns with Thee and Thy Holy Ghost, one God now and forever.
Amen