In the early church, Mary, the mother of Jesus, was always held in the greatest respect, though the chief emphasis lay on her relationship to Eve: just as Eve was involved in the fall, so Mary was involved in our salvation.
In the 4th and 5th centuries, debates about the person of Christ had an effect on the position of Mary in Christian devotion as well. A clear affirmation of Christ’s full humanity was accompanied by enhanced respect for Mary as “Mother of God”. It was in this context that a number of feasts of Mary grew up, including the celebration of her birth. The feast was widely observed in the church, and was retained in the Book of Common Prayer at the Reformation.
There is no known reason why 8 September should have been chosen for the feast. Nothing reliable is known about the birth of Mary or about her parents. Various 2nd and 3rd century documents, written with the pious intention of providing additional details about the birth and early years of Jesus, also contain information about Mary, though none of it is historically reliable. It is from these that most of the stories and traditions about Mary are supplied. But she was Blessed among women and the Mother of the Lord and we don't celebrate the Queen's Birthday or Jesus our Lord's on their real birthday's either.