This Feast was instituted by Pope Pius XI in 1931, on the 15th centenary of the Council of Ephesus, where the dogma of Mary as mother of God was vindicated against the Nestorians.
Ecce Virgo concipiet, et pariet Filium, et vocabitur nomen ejus Emmanuel. * Cantate Domino canticum novum, quia mirabilia fecit.
Behold a Virgin shall conceive and bear a son: and his name shall be called Emmanuel. * Sing ye to the Lord a new canticle: because He hath done wonderful things.
(Isaiah 7:14 and Psalm 97:1 from the Introit of Mass)
Deus, qui de beatae Maríae Vírginis utero Verbum tuum, Angelo nuntiante, carnem suscípere voluísti: praesta supplícibus tuis; ut, qui vere eam Genitrícem Dei credimus, ejus apud te intercessionibus adjuvemur.
O God who didst will that Thy Word should take flesh, at the message of an angel, in the womb of the blessed Virgin Mary; grant unto us Thy suppliants that we who believe her to be indeed the Mother of God may be aided by her intercession with Thee.
Maternitas tua, Dei Genitrix Virgo, gaudium annuntiavit universo mundo: ex te enim ortus est sol justltiae, Christus Deus noster.
Thy Maternity, O Mother of God and Virgin, hath announced joy to the whole world : because out of Thee is arisen the sun of justice, Christ our God.
(Antiphon at the Magnificat of 2nd Vespers)From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/10046a.htm