This feast is a commemoration of that happy and joyful day on which the ever-blessed virgin Mother of God first saw the light of day. The Church accordingly sings on this day,
"Thy nativity, virgin Mother of God, has brought joy to the whole world; for from thee has come forth the Sun of justice, Christ the Lord, Who putting away cursing bestowed blessing, and by overcoming death obtained for us life eternal."
In the Introit of the Mass the Church sings:
"Hail, holy parent, who as a happy mother brought forth the King Who rules heaven and earth from eternity to eternity. My heart hath uttered a good word; I speak my works to the King."
Glory be to the Father, and to the Son, and to the Holy Ghost. As it was in the beginning, is now, and ever shall be, world without end. Amen.
Vouchsafe, O Lord, we beseech Thee, unto us Thy servants the gift of Thy heavenly grace, that, as in the childbirth of the Blessed Virgin our salvation began, so from the votive solemnity of her nativity we may obtain an increase of peace. Through Our Lord Jesus Christ, Amen.
Courtesy of: Catholic-Forum