I wrote this in 2010; I reprint it, together with its admirable thread.
On Saturday 27 November 1830, a young French nun, (S) Catherine Laboure, beheld her second and third visions of the Mother of God in the Sanctuary of her Convent Chapel in the Rue du Bac in Paris. Our Lady appeared to her, radiant, standing on a globe, and with her arms stretched out in a compassionate gesture. From her fingers rays of light fell upon the globe at her feet. An oval frame then formed around her with gold lettering that read: O Mary, conceived without sin, pray for us who have recourse to thee. Our Lady promised great graces to those who wore this design with confidence; she showed the Saint the design which now appears on the back of the Miraculous Medal: a large M surmounted by a bar and cross, with two hearts beneath it, one crowned with thorns, the other pierced with a sword, all encircled by twelve stars.
In 1836, Abbe Desgenettes, who had taken over the Church of Our Lady of Victories (a church degraded and desecrated during the Revolution and with a minute congregation), dedicated his parish to the Immaculate Heart of Mary, and founded a Confraternity of Prayer, which had the Miraculous Medal as its badge. In the days before Newman's conversion, intense prayer was offered for him in this Church by the members of that very same Fraternity. Back in Blighty, it was on the Octave Day of the Assumption in 1845 (a very patrimonial day: it was also the birthday of blessed Edward Bouverie Pusey) that Blessed John Henry Newman first began to wear the Miraculous Medal.
Now back two or three years, to January 20, 1842. On this day, a wealthy Jewish banker called Alphonse Ratisbonne had, in the Church of S Andrea delle Fratte in Rome, a vision of our Lady just as she appeared on the Miraculous Medal. Shunt forward ... please ... to 1847: Newman and St John (who, after their reception, had visited the shrine in Notre Dame des Victoires in thanksgiving for the prayers offered for him there) found themselves now awaiting admission to the presbyterate of the Latin Church, lodged in the Collegio di Propaganda in Rome. Newman makes clear in a number of letters that their windows looked down on the Church of S Andrea delle Fratte; it clearly made some considerable impression upon him. On June 9 1847, his long-time intimate woman friend, Maria Giberne, painted a picture of Newman and St John in a room at Propaganda, with our Lady, as she appears on the Miraculous Medal , between the two of them.
In the Old Missal, in the Appendix pro aliquibus locis, November 27 is the feast of Our Lady Immaculate of the the Miraculous Medal. Let us hope that this commemoration will one day make its way into the Calendar of the Patrimony!