From The Office of Readings:
The present festival, the birth of the Mother of God, is the prelude, while the final act is the fore-ordained union of the Word with flesh. Today the Virgin is born, tended and formed and prepared for her role as Mother of God, who is the universal King of the ages.
Justly, then, do we celebrate this mystery since it signifies for us a double grace. We are led toward the truth, and we are led away from our condition of slavery to the letter of the law. How can this be? Darkness yields before the coming of the light, and grace exchanges legalism for freedom. But midway between the two stands today’s mystery, at the frontier where types and symbols give way to reality, and the old is replaced by the new. Therefore, let all creation sing and dance and unite to make worthy contribution to the celebration of this day. Let there be one common festival for saints in heaven and men on earth. Let everything, mundane things and those above, join in festive celebration. Today this created world is raised to the dignity of a holy place for him who made all things. The creature is newly prepared to be a divine dwelling place for the Creator.
--St. Andrew of Crete--