Of course we have no idea on what date Mary was actually born, and I shall not detain you by speculating on why September 8 was chosen to celebrate this feast. It appears to have been first celebrated liturgically in sixth-century Palestine after the Council of Ephesus acclaiming her as Theotokos or "God-bearer." Since the sources for Mary's origins are apocryphal, Rome was slow to put the feast on the universal calendar; that didn't happen, apparently, until the 8th century, after all the christological councils of the first millennium had done their work. Yet even though the basic outlines of orthodox christology were settled by then, the question of the nature of the Church as Mystical Body of Christ had not been. And while there has been some development, the question still isn't really settled. Most of the theological disputes today, at least in my observation, either stem from or lead to that theme. And so I shall say something about it here. Taking his cues from St Andrew of Crete, A. Valentini has noted:
In the case of all the Saints, the Church commemorates their birthday on the day of their return to the Lord. However, in the cases of St. John the Baptizer and the Blessed Virgin, it also celebrates the day of their earthly birth. This is a singular fact already emphasized in ancient times, for example, by Paschasius Radbertus (d. about 859).
The reason for this fact is not found primarily in the greatness or the privileges of the persons involved but in the singular mission that was theirs in the History of Salvation. In this light, the birth of the Blessed Virgin is considered to be - like that of John the Baptizer - in direct relationship with the coming of the Savior of the world. Thus, the birth and existence of Mary—similar to and even more than those of the Baptizer—take on a significance that transcends her own person. It is explained solely in the context of the History of Salvation, connected with the People of God of the Old Covenant and the New. Mary's birth lies at the confluence of the two Testaments, bringing to an end the stage of expectation and the promises and inaugurating the new times of grace and salvation in Jesus Christ.
Mary, the Daughter of Zion and ideal personification of Israel, is the last and most worthy representative of the People of the Old Covenant but at the same time she is "the hope and the dawn of the whole world." With her, the elevated Daughter of Zion, after a long expectation of the promises, the times are fulfilled and a new economy is established (LG 55).
The birth of Mary is ordained in particular toward her mission as Mother of the Savior. Her existence is indissolubly connected with that of Christ: it partakes of a unique plan of predestination and grace. God's mysterious plan regarding the incarnation of the Word embraces also the Virgin who is His Mother. In this way, the Birth of Mary is inserted at the very heart of the History of Salvation (emphasis added).
Valentini cites Vatican II's Dogmatic Constitution on the Church, Lumen Gentium, which was concerned to present Mary above all as "Mother of the Church." Within the Church, Mary represents the feminine principle in which we all share as members of the Mystical Body of Christ, i.e. the Church. The Church is one body with Christ because she is the Bride whom he, the Bridegroom, has married. Thus Mary is Spouse of the Holy Spirit as well as Mother of the Son. As her divine Son's most perfect human disciple, filled to overflowing with the Holy Spirit who conceived the Son in her, she leads to the Son by the power of the Spirit all those who seek her care and protection. As personification of the Church, she helps to make the Church as collectivity what she is individually: the receiver and bearer of God in the world. Such is the "Marian charism" of all believers.
At the other pole of the Church we have the clerical hierarchy, with the college of bishops as collective head and the pope as head of them. The hierarchy represents the masculine principle of headship and self-immolation that is fully found only in the life, death, resurrection, and divine person of God the Son Incarnate: Our Lord Jesus Christ. As sinners, they fulfill their role quite imperfectly even as Mary fulfills hers perfectly. They and their role exist for the sake of all of us, whom the human person Mary represents. Such is the balance of the Church. The representation of headship, through which Jesus Christ exercises his headship, is done by sinful men; the representation of submission and sanctification, which Jesus also uses for his purpose, is done by a sinless woman.
We need to meditate more on such truths today. I could go on, but I don't have time and for now this is enough food for thought. I offer it to you, my readers, and to God for further development.