Ecce, Virgo concipiet, et pariet filium, et vocabitur nomen ejus Emmanuel.
Behold, the virgin shall be with child and shall bear a son, and he shall be called Emanuel.
Cantate Domino canticum novum: quia mirabilia fecit.
Sing the Lord a new song, a song of wonder at his doings.
Bl Pope John XXIII opened the Second Vatican Council on this Marian feast with these words:
"Mother Church rejoices that, by the singular gift of Divine Providence, the longed-for day has finally dawned when--under the auspices of the virgin Mother of God, whose maternal dignity is commemorated on this feast--the Second Vatican Ecumenical Council is solemnly opened beside Saint Peter's tomb."
The Feast was instituted by Pope Pius XI in 1931 for the whole Latin Church, on the occasion of the 15th centenary of the Council Ephesus, in 431, which proclaimed the Virgin Mary Theotokos, the God-bearer. The term was adopted as a way to assert the Divinity of Christ from which it follows that what is predicated of Christ is predicated of God. So, if Mary is the mother of Jesus, she is the Mother of God.
The Feast was moved to the 1st of January in the Ordinary Form of the Roman Rite, it remains on this day in the Extraordinary Form.