The Birth of the Virgin by Giotto (+1337AD)
You can read about the history of this feast day here: Old Catholic Encyclopedia on Nativity of Our Lady.
This site has a bit more about the feast day, as well: From Goffine's Devout Instructions
On the occasion of the feast, here is a setting of the Magnificat by Spanish-born Hernando Franco (+1585AD) who would be musically active in Guatemala and Mexico:
Finally, it seems fitting to quote from the Encyclical Letter of Pope St. Pius X, Ad Diem Illum Laetissimum, published in 1904. He writes, in part:
"10. For is not Mary the Mother of Christ? Then she is our Mother also. And we must in truth hold that Christ, the Word made Flesh, is also the Savior of mankind. He had a physical body like that of any other man: and again as Savior of the human family, he had a spiritual and mystical body, the society, namely, of those who believe in Christ. "We are many, but one sole body in Christ" (Rom. xii., 5). Now the Blessed Virgin did not conceive the Eternal Son of God merely in order that He might be made man taking His human nature from her, but also in order that by means of the nature assumed from her He might be the Redeemer of men. For which reason the Angel said to the Shepherds: "To-day there is born to you a Savior who is Christ the Lord" (Luke ii., 11). Wherefore in the same holy bosom of his most chaste Mother Christ took to Himself flesh, and united to Himself the spiritual body formed by those who were to believe in Him. Hence Mary, carrying the Savior within her, may be said to have also carried all those whose life was contained in the life of the Savior. Therefore all we who are united to Christ, and as the Apostle says are members of His body, of His flesh, and of His bones (Ephes. v., 30), have issued from the womb of Mary like a body united to its head. Hence, though in a spiritual and mystical fashion, we are all children of Mary, and she is Mother of us all. Mother, spiritually indeed, but truly Mother of the members of Christ, who are we (S. Aug. L. de S. Virginitate, c. 6).
11. If then the most Blessed Virgin is the Mother at once of God and men, who can doubt that she will work with all diligence to procure that Christ, Head of the Body of the Church (Coloss. i., 18), may transfuse His gifts into us, His members, and above all that of knowing Him and living through Him (I John iv., 9)?
12. Moreover it was not only the prerogative of the Most Holy Mother to have furnished the material of His flesh to the Only Son of God, Who was to be born with human members (S. Bede Ven. L. Iv. in Luc. xl.), of which material should be prepared the Victim for the salvation of men; but hers was also the office of tending and nourishing that Victim, and at the appointed time presenting Him for the sacrifice. Hence that uninterrupted community of life and labors of the Son and the Mother, so that of both might have been uttered the words of the Psalmist"My life is consumed in sorrow and my years in groans" (Ps xxx., 11). When the supreme hour of the Son came, beside the Cross of Jesus there stood Mary His Mother, not merely occupied in contemplating the cruel spectacle, but rejoicing that her Only Son was offered for the salvation of mankind, and so entirely participating in His Passion, that if it had been possible she would have gladly borne all the torments that her Son bore (S. Bonav. 1. Sent d. 48, ad Litt. dub. 4). And from this community of will and suffering between Christ and Mary she merited to become most worthily the Reparatrix of the lost world (Eadmeri Mon. De Excellentia Virg. Mariae, c. 9) and Dispensatrix of all the gifts that Our Savior purchased for us by His Death and by His Blood.
13. It cannot, of course, be denied that the dispensation of these treasures is the particular and peculiar right of Jesus Christ, for they are the exclusive fruit of His Death, who by His nature is the mediator between God and man. Nevertheless, by this companionship in sorrow and suffering already mentioned between the Mother and the Son, it has been allowed to the august Virgin to be the most powerful mediatrix and advocate of the whole world with her Divine Son (Pius IX. Ineffabilis). The source, then, is Jesus Christ "of whose fullness we have all received" (John i., 16), "from whom the whole body, being compacted and fitly joined together by what every joint supplieth, according to the operation in the measure of every part, maketh increase of the body unto the edifying of itself in charity" (Ephesians iv., 16). But Mary, as St. Bernard justly remarks, is the channel (Serm. de temp on the Nativ. B. V. De Aquaeductu n. 4); or, if you will, the connecting portion the function of which is to join the body to the head and to transmit to the body the influences and volitions of the head -- We mean the neck. Yes, says St. Bernardine of Sienna, "she is the neck of Our Head, by which He communicates to His mystical body all spiritual gifts" (Quadrag. de Evangel. aetern. Serm. x., a. 3, c. iii.).
14. We are then, it will be seen, very far from attributing to the Mother of God a productive power of grace -- a power which belongs to God alone. Yet, since Mary carries it over all in holiness and union with Jesus Christ, and has been associated by Jesus Christ in the work of redemption, she merits for us "de congruo," in the language of theologians, what Jesus Christ merits for us "de condigno," and she is the supreme Minister of the distribution of graces. Jesus "sitteth on the right hand of the majesty on high" (Hebrews i. b.). Mary sitteth at the right hand of her Son -- a refuge so secure and a help so trusty against all dangers that we have nothing to fear or to despair of under her guidance, her patronage, her protection. (Pius IX. in Bull Ineffabilis).
15. These principles laid down, and to return to our design, who will not see that we have with good reason claimed for Mary that -- as the constant companion of Jesus from the house at Nazareth to the height of Calvary, as beyond all others initiated to the secrets of his Heart, and as the distributor, by right of her Motherhood, of the treasures of His merits,-she is, for all these reasons, a most sure and efficacious assistance to us for arriving at the knowledge and love of Jesus Christ. Those, alas! furnish us by their conduct with a peremptory proof of it, who seduced by the wiles of the demon or deceived by false doctrines think they can do without the help of the Virgin. Hapless are they who neglect Mary under pretext of the honor to be paid to Jesus Christ! As if the Child could be found elsewhere than with the Mother!
16. Under these circumstances, Venerable Brethren, it is this end which all the solemnities that are everywhere being prepared in honor of the holy and Immaculate Conception of Mary should have in view. No homage is more agreeable to her, none is sweeter to her than that we should know and really love Jesus Christ. Let then crowds fill the churches -- let solemn feasts be celebrated and public rejoicings be made: these are things eminently suited for enlivening our faith. But unless heart and will be added, they will all be empty forms, mere appearances of piety. At such a spectacle, the Virgin, borrowing the words of Jesus Christ, would address us with the just reproach: "This people honoureth me with their lips, but their heart is far from me" (Matth. xv., 8).
17. For to be right and good, worship of the Mother of God ought to spring from the heart; acts of the body have here neither utility nor value if the acts of the soul have no part in them. Now these latter can only have one object, which is that we should fully carry out what the divine Son of Mary commands. For if true love alone has the power to unite the wills of men, it is of the first necessity that we should have one will with Mary to serve Jesus our Lord. What this most prudent Virgin said to the servants at the marriage feast of Cana she addresses also to us: "Whatsoever he shall say to you, do ye" (John ii., 5).
Now here is the word of Jesus Christ: "If you would enter into life, keep the commandments" (Matt. xix., 17). Let them each one fully convince himself of this, that if his piety towards the Blessed Virgin does not hinder him from sinning, or does not move his will to amend an evil life, it is a piety deceptive and Iying, wanting as it is in proper effect and its natural fruit.
18. If anyone desires a confirmation of this it may easily be found in the dogma of the Immaculate Conception of Mary. For leaving aside tradition which, as well as Scripture, is a source of truth, how has this persuasion of the Immaculate Conception of the Virgin appeared so conformed to the Catholic mind and feeling that it has been held as being one, and as it were inborn in the soul of the faithful? "We shrink from saying," is the answer of Dionysius of Chartreux, "of this woman who was to crush the head of the serpent that had been crushed by him and that Mother of God that she had ever been a daughter of the Evil One" (Sent. d. 3, q. 1). No, to the Christian intelligence the idea is unthinkable that the flesh of Christ, holy, stainless, innocent, was formed in the womb of Mary of a flesh which had ever, if only for the briefest moment, contracted any stain. And why so, but because an infinite opposition separates God from sin? There certainly we have the origin of the conviction common to all Christians that Jesus Christ before, clothed in human nature, He cleansed us from our sins in His blood, accorded Mary the grace and special privilege of being preserved and exempted, from the first moment of her conception, from all stain of original sin.
19. If then God has such a horror of sin as to have willed to keep free the future Mother of His Son not only from stains which are voluntarily contracted but, by a special favor and in prevision of the merits of Jesus Christ, from that other stain of which the sad sign is transmitted to all us sons of Adam by a sort of hapless heritage: who can doubt that it is a duty for everyone who seeks by his homage to gain the heart of Mary to correct his vicious and depraved habits and to subdue the passions which incite him to evil?
20. Whoever moreover wishes, and no one ought not so to wish, that his devotion should be worthy of her and perfect, should go further and strive might and main to imitate her example. It is a divine law that those only attain everlasting happiness who have by such faithful following reproduced in themselves the form of the patience and sanctity of Jesus Christ: "for whom He foreknew, He also predestined to be made conformable to the image of His Son; that He might be the first-born amongst many brethren" (Romans viii., 29). But such generally is our infirmity that we are easily discouraged by the greatness of such an example: by the providence of God, however, another example is proposed to us, which is both as near to Christ as human nature allows, and more nearly accords with the weakness of our nature. And this is no other than the Mother of God. "Such was Mary," very pertinently points out St. Ambrose, "that her life is an example for all." And, therefore, he rightly concludes: "Have then before your eyes, as an image, the virginity and life of Mary from whom as from a mirror shines forth the brightness of chastity and the form of virtue" (De Virginib. L. ii., c. ii.)
21. Now if it becomes children not to omit the imitation of any of the virtues of this most Blessed Mother, we yet wish that the faithful apply themselves by preference to the principal virtues which are, as it were, the nerves and joints of the Christian life -- we mean faith, hope, and charity towards God and our neighbor. Of these virtues the life of Mary bears in all its phases the brilliant character; but they attained their highest degree of splendor at the time when she stood by her dying Son. Jesus is nailed to the cross, and the malediction is hurled against Him that "He made Himself the Son of God" (John xix., 7). But she unceasingly recognized and adored the divinity in Him. She bore His dead body to the tomb, but never for a moment doubted that He would rise again. Then the love of God with which she burned made her a partaker in the sufferings of Christ and the associate in His passion; with him moreover, as if forgetful of her own sorrow, she prayed for the pardon of the executioners although they in their hate cried out: "His blood be upon us and upon our children" (Matth. xxvii., 25)."