J.M.J. On the previous Liturgical Calendar before the one that is currently in use (but is still used in the Extraordinary Form of the Mass), tomorrow is the Feast of the Divine Maternity of Mary. (January 1st is the Solemnity of the Blessed Virgin Mary, the Mother of God for the Ordinary Form of the Mass.)
The Reverend Gabriele Maria Roschini, O.S.M., the great Italian Mariologist, wrote in Summula Mariologiae: Scholis Theologicis accommodata (Rome: Angelus Belardetti, 1952): "Mary, therefore, since she is truly the Mother of Jesus, Who is the true God, is truly and properly the Mother of God." (page 69)
How did the Church come to this awareness?
Again, Father Roschini: "The Divine Maternity of Mary was solemnly defined, against Nestorius, in the Council of Ephesus in the year 431, special joy of all the Faithful. The true nature and foundations of the Divine Maternity were fully explained by Pius XI in the Encyclical Lux veritatis." (ibid.)
"Holy Mary, Mother of God, pray for us sinners now and at the hour of our death. Amen."