Which is one of the three birthdays Holy Church celebrates, the other two being the Nativity of Saint John Baptist in June and of course that of Our Blessed Lord at Christmas.
The origin of the feast of the Virgin Mary’s Nativity is a matter of speculation, and the reason for the choice of date is unknown. It was celebrated at Constantinople by the 530s, when St Romanus the Melodist composed a hymn for it; by the seventh century, it had passed to the West, and Pope St Sergius I (687-701) decreed that it be should celebrated with a procession from the church of St Adrian (who shares his feast day with the Birth of the Virgin) to St Mary Major. It would seem, however, that it was rather slower to be accepted than the other early Marian feasts, the Purification, Annunciation and Assumption, since it is not mentioned in some important early liturgical books. Thus we find it included in the oldest manuscript of the Gelasian Sacramentary in roughly 750 A.D., but missing from the calendar in some later books. The liturgical commentators of the High Middle Ages such as Sicard of Cremona and William Durandus were aware that it was of later institution.
And because it was 'more slowly accepted in the West' it gained a vigil only in some places and not at Rome itself (and hence not in the Roman Missals subsequent to the Pian edition in 1570). But in the Ambrosian Rite at Milan the vigil is celebrated today, as Dr DiPippo's essay at New Liturgical Movement, from which I've quoted supra, describes.
The Mass in the Ambrosian Rite, if I've understood correctly, employs this blended text, taken from the Books of the Song of Songs and from Ecclesiasticus, as the lesson for both the Vigil and at the Mass of the day itself; I believe that such a thing doesn't happen in the Roman Rite.
Thus sayeth Wisdom: Canticum 6, 8-9 She is the only one of her mother, the chosen of her that bore her. The daughters saw her, and declared her most blessed: the queens and concubines, and they praised her. Who is she that cometh forth as the morning rising, fair as the moon, bright as the sun, terrible as an army set in array? Ecclus 24, 24-28 I am the mother of fair love, and of fear, and of knowledge, and of holy hope. In me is all grace of the way and of the truth, in me is all hope of life and of virtue. Come over to me, all ye that desire me, and be filled with my fruits. For my spirit is sweet above honey, and my inheritance above honey and the honeycomb. My memory is unto everlasting generations.