In 431 AD, Emperor Theodosius II convened the Council of Ephesus bringing together all of Christendom to tackle Nestorius, the then Patriarch of Constantinople and his arguments that Mary should be called Christotokos instead of Theotokos. The results of the council were clear - Nestorius and his followers (Nestorians) were considered heretics and excommunicated and the Nicaean Creed was reaffirmed. The title of Theotokos was so very important because it clearly delineated the Divine and Human nature of Christ, rather than the simply human nature that Christotokos implied. From Θεοτόκος (Theotokos) Mater Dei is rendered (literally, Mother of God). Another popular rendering is Πλατυτέρα τῶν οὐρανῶν - Platytera ton Ouranon, meaning More spacious than the Heavens.
In the Early Church at Rome, a feasted called the Natale of the Mother of God was celebrated until it began to be overshadowed by the feasts of the Annunciation and the Assumption that were adopted from Constantinople in the 7th century. The Octave of Christmas as it began to be called (the 8th day of Christmastide) was celebrated as the day Christ was circumcised and given the name Jesus. During the 13th and 14th centuries, Rome began to celebrate this day with the feast of the Circumcision of the Lord and the Octave of the Nativity but retained an orientation towards Mary, retaining antiphons and responsories glorifying the maternity of the Blessed Ever Virgin Mary.
In 1751, King Joseph Manuel had his petition approved to celebrate the Feast of the Maternity of the Blessed Virgin Mary together with the Purity of Mary on the first Sunday in May throughout Portugal, Brazil, and Algeria. Soon afterwards, in 1778, Naples began to celebrate and Tuscany as well in 1807. The Feast was extended to the entirety of the Catholic Church in 1931 by Pope Pius XI.
The 1969 revision of the Roman General Calendar removed the feast from October 11th, stating:
"The Maternity of the Blessed Virgin Mary is celebrated on 1 January in the Solemnity of Mary, the Mother of God."
In Pope Paul VI's Apostolic letter Marialis Cultus, we can find the following:
"This celebration, placed on January 1 ...is meant to commemorate the part played by Mary in this mystery of salvation. It is meant also to exalt the singular dignity which this mystery brings to the "holy Mother...through whom we were found worthy to receive the Author of life." It is likewise a fitting occasion for renewing adoration of the newborn Prince of Peace, for listening once more to the glad tidings of the angels (cf. Lk. 2:14), and for imploring from God, through the Queen of Peace, the supreme gift of peace."
While always celebrated on January 1st in the Roman Catholic church, other rites celebrate the feast on a different day - the Byzantine Rite and West/East Syriac rites celebrate Mary, Mother of God on December 26th and the Coptic Church on January 16th. The Eastern Orthodox Church with the Anglican Communion and Lutheran churches instead celebrate the Feast of the Circumcision of Christ on January 1st.