This feast originated in the East where it is mentioned very early. At Rome it was not observed before the reign of Pope Sergius I. As Eve our first mother, arose from the side of Adam, dazzling with life and innocence, so Mary came forth, bright and immaculate from the heart of the eternal Word, Who, by the co-operation of the Holy Ghost, as the Liturgy teaches us, was pleased to form that body and soul which were to be, one day, his tabernacle and altar. This is the sublime meaning of the feast of the Birthday of the Blessed Virgin Mary. It is the dawn foretelling the day which already breaks behind the eternal hills, the mystic rod which springs from Paradise; it is the symbolic fleece which is stretched on our dry earth to catch the miraculous dew. This is the new Eve, that is to say, the life and the Mother of all the living, who is born today for those to whom the first Eve became the mother of sin and death.
(Source: The New Roman Missal, Rev. F. X. Lasance)