On July 18th, 1830, Saint Catherine Laboure stated that she woke up that night hearing a child's voice coming from the chapel, asking her to come. She climbed out of bed and followed the voice where she had a vision of Mary standing in the chapel waiting for her. During the conversation, Mary said to her
"My child, I am going to give you a mission"
On November 27th, 1830, Saint Catherine reported that Mary came to her during evening prayers. Standing inside an oval frame and upon a globe, Mary wore several rings that shone rays of light all over the globe. Around the globe the words came into sight:
Ô Marie, conçue sans péché, priez pour nous qui avons recours à vous
"O Mary, conceived without sin, pray for us who have recourse to thee"
The frame began to rotate, and soon Saint Catherine could see a circle of twelve starts with the letter M surmounted by a cross and the Sacred Heart of Jesus crowned with thorns next to the Immaculate Heart of Mary pierced with a sword. Mary then spoke aloud to Saint Catherine:
"Have a medal struck upon this model. Those who wear it will receive great graces, especially if they wear it around the neck"
As the vision faded, Saint Catherine immediately set off for her confessor who explained in detail everything she had seen and heard. The priest watched her behavior for two years, investigating and questioning what had happened before taking the information to his archbishop. He at the time did not reveal that it was Saint Catherine who had the visions. The request to begin creating and blessing the medallions was quickly approved and goldsmith Adrien Vachette began making them.
For more information, including requesting a free medal, please visit the Association of the Miraculous Medal