The birth of the Blessed Virgin Mary has been celebrated as a liturgical feast at least from the sixth century. Its origin can be traced to the occasion of the consecration of a church in Jerusalem just inside St. Stephen’s Gate, near the Pool of Bethesda, on the traditional site of the house of Ss. Joachim and Anne. Within a few years the liturgy was celebrated in Rome, having been introduced by monks from the East, and the celebration included a procession to the Basilica of St. Mary Major.
Although the actual date of Mary’s birth isn’t known, the Church settled on September 8th, and the celebration Solemnity of the Immaculate Conception was fixed on December 8th, as the date corresponding to nine months before the celebration of her Nativity.
These two feasts can be seen as a kind of bridge between the Old Testament and the New Testament. With the conception and birth of the Blessed Virgin, God completed the new Ark – the living Temple – in which He would dwell. Through Mary, Jesus the Incarnate God has come to us.
O Lord, we beseech thee, bestow on thy servants the gift of heavenly grace: that as our redemption began to dawn in the child-bearing of the Blessed Virgin Mary; so this festival of her Nativity may yield us an increase of peace; through Jesus Christ thy Son our Lord, who liveth and reigneth with thee, in the unity of the Holy Spirit, ever one God, world without end. Amen.