The Nativity of the Blessed Virgin Mary: from East to West

Today is the Feast of the Nativity of the Blessed Virgin Mary in the Roman and Eastern Catholic Church, called the Feast of the Nativity of the
Theotokos (the God-bearer) in the East, including in the Orthodox Churches. In fact, this birthday feast originated in the East in the 6th century and did not come to the Western Church until sometime in the 7th century, not becoming more common in the 8th.

The date for her birth was chosen because it is at the beginning of the Liturgical Year in the East (the Liturgical Year in the West begins with the First Sunday of Advent).  In this case, "East" means both Eastern-Rite (or Byzantine) Catholic AND Orthodox.

Since her birth is celebrated on September 8, her Immaculate Conception is celebrated on December 8--nine months before her birth. In the Eastern-Rite Catholic Church the Feast of the Conception of the Mother of God is celebrated on December 8, and has been for centuries. As the Byzantine Catholic Archeparchy of Pittsburgh explains:

Already at the beginning of the liturgical year, on September 8th, we celebrate the Feast of the Nativity of the Mother of God, since Mary’s birth signalled “the beginning of our salvation” (cf. Stich era of Utia). In connection with Mary’s birth, since the eighth century, we celebrate the Feast of the Conception of the Mother of God, recently referred to as the Immaculate Conception (cf. Byzantine Leaflet Series, n. 36).

Saint Augustine of Hippo rejoiced on the Feast of the Nativity the Blessed Virgin Mary according to this blog:

On today’s feast of the Blessed Virgin Mary, St. Augustine writes in the old Divine Office: “Dearly beloved brethren, the day for which we have longed, the Feast-day of the Blessed and venerable and Ever-Virgin Mary, that day is come. Let our land laugh and sing with merriment, bathed in the glory of this great Virgin’s rising. She is the flower of the fields on which the priceless lily of the valleys hath blossomed. This is she whose delivery changed the nature that we draw from our first parents, and cleansed away their offense.” . . .

Father David Nix explains why Saint Augustine says that Mary's birth "changed the nature that we draw from our first parents":

The birthday of Mary is in some way the birthday of a new humanity since Jesus got His humanity from her and since we are adopted into his family through baptism. The waters of St. Anne breaking were in some since a prefigurement of the waters of our baptism, our new family. These graces of new life do not originate in Mary, but they flow through her, as Christ came through her once. When we are baptized, we are divinized by God Himself transforming us into tabernacles of the Blessed Trinity. But we also enter a new humanity that Jesus took from Mary, even though He—God— created her most perfect soul, celebrated on the Immaculate Conception (8th of December.) But 9 months later, we have the full flowering of this conception on her birthday, on the 8th of September, today.

Saint Andrew of Crete (8th century) offers a similar meditation here.

More about the Feast of the Nativity of the Theotokos from the Orthodox Church in America website here.

Image Source (Public Domain): 12-century German Nativity of Mary with her father, Joachim wearing a Jewish hat