I'm not an avid reader of Bl. Anna Catherine Emmerich, but I find the narration of Our Lady's Nativity to be charming and edifying - especially aspects related to the eve of Our Lady's birth - which also may help explain the Order of Carmel's special devotion to Our Lady's Nativity.
EVE OF MARY'S BIRTH
What gladness throughout all nature! Birds are singing, lambs and kids are gamboling, and swarms of doves are fluttering with joy around the spot upon which once stood Anne's abode. I see only a wilderness there at the present day. But I had a vision of pilgrims in the far-off times who, girded and with long staves in their hands, wended their way through the country to Mount Carmel. On their head they wore a covering wound around like a turban. They too participated in the joy of nature. And when in their astonishment they asked the hermits that dwelt in the neighborhood the cause of this remarkable exultation, they received for answer, that such manifestations of gladness were customary. They were always observed upon the eve of the anniversary of Mary's birth around that spot where once stood Anne's house. The hermits told them of a holy man of the early times who had been the first to notice these wonders in nature. His account gave rise to the celebration of the feast of Mary's Nativity which soon became general throughout the Church. And now I too beheld how this came to pass.
I saw a pious pilgrim, two hundred and fifty years after Mary's death, traversing the Holy Land, visiting and venerating all places connected with the actions of Jesus while on earth. He was supernaturally guided. Sometimes he tarried several days together in certain places in which he tasted extraordinary consolation. There he prayed and meditated, and there also he received revelations from On High. For several years he had, from the 7th to the 8th of September, noticed a great jubilation in nature and heard angelic voices singing in the air. He prayed earnestly to know the meaning of all this, and it was made known to him in a vision that that was the birthnight of the Blessed Virgin Mary. He was on his way to Mount Sinai when he had this vision. In it he was informed also of the existence of a chapel built in Mary's honor in a cave of the Prophet Elias.
I saw him again when he arrived at the mount. Where the convent now stands there dwelt even at that early period, hermits scattered here and there. It was then as inaccessible from the valley as it is now. To reach the top of the mountain from that side, hoisting machines were used. I saw that in consequence of the pilgrim's communication, the 8th of September was here first celebrated in the year 250, and that later it was introduced into other parts of the Church.
I learned from St. Bridget that if pregnant women fast on the eve of Mary's birth and say fervently nine Hail Mary's to honor the nine months she passed in Anne's womb; if they frequently repeat these prayers during their pregnancy, and especially on the eve of their delivery, receiving then the holy Sacraments devoutly, she will offer their prayer to God herself and bring them through even very critical circumstances to a happy delivery.
I saw the Blessed Virgin on the eve of her nativity. She said to me: "Whoever says this evening" (Sept. 7th) "nine times the Hail Mary lovingly and devoutly to honor the nine months spent in my mother's womb as also my birth, and continues the same devotion for nine consecutive days, daily gives to the angels nine flowers for a bouquet. This bouquet they bear to heaven and offer to the Most Holy Trinity to obtain some favor for the one that prays." - Source