January 1 is the Solemnity of St. Mary the Mother of God.
Why do we call St. Mary the Mother of God? We do so because we believe that Jesus is truly and fully God as well as truly and fully man. The great Church Council of Ephesus in 431 proclaimed this truth, expressing the orthodox faith of Christians in the mystery of Christ, that He is one Divine Person who is both true God and true man.
The Nestorians of that time were preaching that St. Mary was the Mother of Jesus but not the Mother of God. What the Council of Ephesus proclaimed was in part to counter the false belief of the Nestorians and to promulgate the catholic (that is, universal and reliable) belief about the nature of Christ and his Blessed Mother. St. Mary had the singular privilege of giving birth to the one who is both God and man, Jesus Christ, and hence she rightly can be called the "Theotokos," often translated as "She who gave birth to the One who is God."
The feast day was celebrated in the East before the West, but was widespread by the 5th century. It has a bit of a convoluted history. It was first celebrated on January 1 as the Maternity of the Blessed Mary. But by the 14th century that day was used for the Feast of the Circumcision, and the Feast of the Maternity of Mary was moved to October 11. In 1974, the Feast of the Circumcision of Jesus was removed and the feast of the Maternity of Mary was moved back to January 1 and given its current name and status.
Why do we call St. Mary the Mother of God? We do so because we believe that Jesus is truly and fully God as well as truly and fully man. The great Church Council of Ephesus in 431 proclaimed this truth, expressing the orthodox faith of Christians in the mystery of Christ, that He is one Divine Person who is both true God and true man.
The Nestorians of that time were preaching that St. Mary was the Mother of Jesus but not the Mother of God. What the Council of Ephesus proclaimed was in part to counter the false belief of the Nestorians and to promulgate the catholic (that is, universal and reliable) belief about the nature of Christ and his Blessed Mother. St. Mary had the singular privilege of giving birth to the one who is both God and man, Jesus Christ, and hence she rightly can be called the "Theotokos," often translated as "She who gave birth to the One who is God."
The feast day was celebrated in the East before the West, but was widespread by the 5th century. It has a bit of a convoluted history. It was first celebrated on January 1 as the Maternity of the Blessed Mary. But by the 14th century that day was used for the Feast of the Circumcision, and the Feast of the Maternity of Mary was moved to October 11. In 1974, the Feast of the Circumcision of Jesus was removed and the feast of the Maternity of Mary was moved back to January 1 and given its current name and status.