The birth of the Blessed Virgin Mary has been celebrated since at least the sixth century. This is not a historical feast, of course, and the date was chosen to correspond to the beginning of the Church year in the Eastern Church. The date of her birth also helped to determine the date of the Immaculate Conception – on December 8, nine months prior to today.
We don't have any historical account of the birth of Mary for several reasons. First, births were not recorded at that time very well, in fact we don't have an accurate date for the birth of Jesus. And second, no one would have known at the time of Mary's birth her significance in God's plan for salvation. But that significance does come to light prominently in the tradition that has grown up around her birth. It is said that Mary's parents, Joachim and Anne, are infertile, and they pray for a child. They receive the promise of a daughter who would be central to the salvation of the world.
Every single birth is a sign of hope in our world, and therefore a cause for great celebration. Even though the world may be in a bad place, and dark from sin, birth brings joy because it is a sign of God's wanting the world to continue to bring salvation to all people. Mary's birth stands out prominently among us because of the grace she received from God who chose her to be mother of His Son.
The Byzantine Church Daily Worship proclaims well the joy that we have on this feast of Mary's birth: "Today the barren Anna claps her hands for joy, the earth radiates with light, kings sing their happiness, priests enjoy every blessing, the entire universe rejoices, for she who is queen and the Father's immaculate bride buds forth from the stem of Jesse."
Pray for us, O Holy Mother of God, that we may be made worthy of the promises of Christ.