On November 27, 1830, the Blessed Virgin Mary appeared to St. Catherine Laboure in Paris, France and gave her a vision of an image, with instructions to strike the image, front and back, onto a medal. With this medal Our Lady promised that, "All those who wear it will receive great graces; these graces will be abundant for those who wear it with faith.” The medal bears an image of Our Lady standing on the world, the serpent crushed under her feet, with arms extended and graces pouring forth from her hands. On the reverse is a cross surmounted by the letter 'M', and beneath it the side-by-side symbols of the Sacred Heart of Jesus and Immaculate Heart of Mary, all encircled by twelve stars. The medal symbolizes Mary's perfect spiritual union with Jesus' redemptive mission, and, consequently, her intercessory role in salvation history as the mediatrix of God's graces to mankind through her Son. The medal spread widely and became a source of many graces and miracles for its wearers, earning the name "Miraculous Medal." It is traditionally worn around the neck and has become a treasured sacramental of the Catholic Church. Our Lady of the Miraculous Medal's feast day is November 27th.

Source:  Morning Offering - November 27 Saint of the Day post and podcast