From Saint Boniface Catholic Church website - The angel said to [Mary], "The holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called holy, the Son of God." Luke 1:35. The Solemnity of Mary, Holy Mother of God is a liturgical celebration observed on January 1st which falls exactly one week after Christmas, the end of the octave of Christmas... The Blessed Virgin Mary is honored as the Mother of God because she conceived and gave birth to Christ, who is the incarnation of God the Son, the Second Person of the Blessed Trinity.