While December 12 is widely associated with Our Lady of Guadalupe, that is the date of only one of her appearances to St. Juan Diego. She had appeared to him several times before then, first revealing her presence on December 9.
On December 9, St. Juan Diego was on his way to Mass when he noticed something unusual, as The Month: A Catholic Magazine, published in 1885, describes the scene:
It was at early dawn one December morning in 1531 ... that a poor Indian, who had recently been converted to Christianity and had received in baptism the name of Juan Diego, was going on foot from Tolpetbac ... to hear Mass there and attend the catechism. On passing by a hill called the Tepeyac ... his attention was suddenly arrested by a sound, as of many birds singing sweetly and harmoniously, which seemed to come from out a white and resplendent cloud just then hanging over the summit of the hill.
When St. Juan Diego looked at the strange cloud, he heard a voice come from it, followed by a vision of a beautiful lady:
He next heard the delicate voice of a lady calling him by his name and bidding him come up the steep. Juan obeyed in all simplicity without fear or disquiet of any sort and there in the midst of the cloud's splendor he saw a young and beautiful lady who asked him whither he was going so early.
She then revealed herself to St. Juan Diego and told him what to do next:
Know, my much beloved son, that I am the ever Virgin Mary Mother of the true God, who is the Author of life, the Creator of all things and the Lord of Heaven and earth. It is my desire that a temple should be built me in this place, where as thy Mother and that of thy countrymen, I will display my loving mercy and the compassion, which I feel towards the natives of this land and indeed towards all who love and seek me and solicit my protection.
Our Lady then asked St. Juan Diego to tell the bishop this request, which he did after the apparition ended.
She would return four more times, culminating in bestowing the miraculous image on the tilma that has been preserved for nearly 500 years.
When setting the date of St. Juan Diego's feast day, the Church chose December 9, honoring the first day that he saw the Virgin Mary.
