Learned writers argue that the institution of a festal day in the Church in honour of the nativity of St John the Baptist is most ancient, from Augustine's Sermon 292 on the feast of the same holy Forerunner, where he says that she (the Church) received it from the tradition of the elders. From this passage of Augustine it is gathered that the natal day of St John began to be observed long before the Council of Agde, held at the beginning of the sixth century, as Thomassin supposed (Book V, On the Celebration of Feasts, chapter 5). That this day is held among the chief solemnities by the Church, St Bernard testifies in Letter 174:
The Church, which of the rest of the saints marks and proclaims as precious not their nativity but their death, by a certain singular exception bestows festal joys upon this man, and venerates his very birth, concerning whom it is singularly read, with an angel announcing it: and many shall rejoice in his nativity. This first sermon Muratori transcribed and published from an Ambrosian codex. The order of our religion and devotion demands, brethren, that today we rejoice over the nativity of blessed John the Baptist, who, foreordained by God, came to proclaim the gladness of the human race and the joy of heaven.
This is he from whose mouth the world heard, with a new voice, that our Redeemer was present, that the Lamb of God was present; who also, when his parents despaired of offspring, was sent by an angel to be born as a witness, that he might be, as it were, the undoubted herald of so great a mystery. And who would not more prudently believe that he announced divine mysteries, in whose very begetting he might perceive that a heavenly care had been at work? For he, when he was not yet a son and was still carried imperfect in the womb, nevertheless, by the prerogative of the grace granted to him, filled the maternal bowels with eternal joy, and the blessed mother recognized the happiness of a womb long barren before her own delivery. For Elizabeth said to Mary, as has been read:
Behold, as soon as the voice of your greeting reached my ears, the infant leaped for joy in my womb. And whence is this to me, that the Mother of my Lord should come to me (Luke 1)? It is no wonder, most beloved, if this aged woman was enriched with the gift of foreknowledge, she who was to bring forth the herald of the Most High God. She was made more glorious by her very barrenness, who, while her fruitfulness was deferred, obtained by the gift of one birth the honour of all posterity.
She who, as though unfruitful, groaned together with her husband even to their last and deadly old age, suddenly brought forth not for herself alone a son, but for the whole world a herald of eternal salvation; and such a herald that, before he came forth from the womb, by the privilege of his future ministry he imparted a prophetic spirit to his mother's tongue, and by the power of the name foretold to him through the angel unlocked the mouth of his father Zacharias, which unbelief had closed. For Zacharias had fallen silent, not that he might remain dumb, but that, the use of speech being restored to him by God, he might bear heavenly testimony to the prophetic infant.
Therefore the priest, who used to speak to all the people, was struck dumb, so that, since the silence of the priest was public, the mystery of the sacred nativity might come to the knowledge of the whole people, and no one might dare not to believe it. He who as a father had doubted that his son would be born incurred the penalty of silence: for the Evangelist says of him, He was not himself the light, but was to bear witness of the light, that all might believe through him (John 1). He was not indeed the light, but he was wholly in the light, who deserved to bear witness to the true Light.
And therefore, brethren, in honour of most blessed John let us celebrate this day of his nativity with all exultation, who, to drive away the darkness of the world, both first of all recognized the everlasting light of heaven and was the first to point it out.