On the eighth day of Christmas, the Church keeps the Solemnity of Mary, the Holy Mother of God. The title — in Greek, Theotokos, "God-bearer" — was defined by the Council of Ephesus in 431, settling that the one born of Mary is not merely a man joined to God but God himself made flesh. To call Mary the Mother of God is to confess the deepest mystery of the Incarnation: that the eternal Word truly took our nature, that his birth was a true birth, and that the woman who bore him bore God.
The feast falls on the octave day of Christmas, the eighth day, and binds the celebration of Christ's birth to the woman who gave him flesh. It is also a day of prayer for peace.