The Solemnity of Mary, Mother of God is a liturgical feast of the Blessed Virgin Mary, celebrated by the Roman Catholic Church on 1 January, the Octave Day of Christmas. In some countries this day is considered a holy day of obligation.
The feast was celebrated in the east before the west, but by the 5th century it was celebrated in France and Spain on the Sunday before Christmas. In Rome, even before the 7th century, 1 January was used as a celebration of the Maternity of the Blessed Virgin Mary. In the 13th and 14th centuries, the Feast of the Circumcision of Christ had come to replace the Marian feast on 1 January. The celebration of the Feast of the Circumcision on 1 January was expanded to the entire Roman Catholic Church in 1570 when Pope Pius V promulgated the Roman Missal. In 1914, the feast of the "Maternity of the Blessed Virgin Mary" was established in Portugal, occurring on 11 October. In 1931, this feast was extended to the entire Roman Catholic Church by Pope Pius XI and maintained on 11 October. Following the Second Vatican Council in 1974, Pope Paul VI removed the Feast of the Circumcision of Christ from the liturgical calendar, and replaced it with the feast of the "Solemnity of Mary, Mother of God." Traditionalist Catholics continue to celebrate this feast day with the old name "The Maternity of the Blessed Virgin Mary" on 11 October.
The feast is a celebration of Mary's motherhood of Jesus. The title “Mother of God” is a western derivation from the (Greek: Theotokos, the God-bearer). The term “Theotokos” was adopted at the Council of Ephesus 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. Therefore, the title “Mother of God” and the “Solemnity of Mary, Mother of God”, which celebrates her under this title, are at once Mariological and Christological.