The origins of the feast celebrating Mary’s divine maternity are obscure, but there is some evidence of ancient feasts commemorating Mary’s role as theotokos.
Around 500 AD, the Eastern Church celebrated a “Day of the Theotokos” either before or after Christmas. This celebration eventually evolved into a Marian feast on Dec. 26 in the Byzantine calendar and on Jan. 16th in the Coptic calendar.
In the West, Christmas has generally been celebrated with an octave, an eight day extension of the feast. The Gregorian and Roman calendars of the 7th Century mark the Christmas octave day with a strong Marian emphasis. However, eventually in the West, the eighth day of the octave of Christmas was celebrated as the Feast of the Circumcision of Jesus.
The push for an official feast day celebrating Mary’s divine maternity started in Portugal, and in 1751, Pope Benedict XIV allowed Portugal’s churches to celebrate Mary’s divine maternity on the first Sunday in May. The feast was eventually extended to other countries and by 1914 was being celebrated on Oct. 11. The feast of Mary’s divine maternity became a universal feast in 1931.
However, following Vatican II, Pope Paul VI decided to change the feast of Jesus’ Circumcision to the Solemnity of Mary Mother of God to reclaim the ancient Western Marian emphasis at the end of the Octave of Christmas. Celebrating Mary’s divine maternity during the Christmas octave makes complete sense in that the celebration is connected closely to Christ’s birth. Pope Paul VI gave his reasoning for the change:
In the revised arrangement of the Christmas season, we should all turn with one mind to the restored solemnity of the Mother of God. This feast was entered into the calendar in the liturgy of the city of Rome for the first day of January. The purpose of the celebration is to honor the role of Mary in the mystery of salvation and at the same time to sing the praises of the unique dignity thus coming to “the Holy Mother…through whom we have been given the gift of the Author of life.”
This same solemnity also offers an excellent opportunity to renew the adoration rightfully to be shown to the newborn Prince of Peace, as we once again hear the good tidings of great joy and pray to God, through the intercession of the Queen of Peace, for the priceless gift of peace. Because of these considerations and the fact that the octave of Christmas coincides with a day of hope, New Year’s Day, we have assigned to it the observance of the World Day of Peace (Paul VI, Marialis Cultus, Feb. 2, 1974, no.5).
Thus Pope Paul VI highlighted the feast’s celebration of both Mary and Jesus. He also noted the connection to New Year’s Day and Mary’s role as Queen of Peace. January 1st, the Solemnity of Mary Mother of God is also the observed “World Day of Peace.”
There are many Marian feasts in the Church Calendar. These include The Assumption of Mary, The Immaculate Conception, Our Lady of Sorrows, Our Lady of Guadalupe, Our Lady of Consolation, among many others. However, Mary Mother of God focuses on Mary’s divine maternity. – Source: http://www.churchyear.net/motherofgod.html