The Miraculous Medal

The devotion commonly known as that of the Miraculous Medal owes
its origin to Zoe Labore, a member of the Daughters of Charity of
St. Vincent de Paul, known in religion as Sister Catherine (She was
subsequently canonized), to whom the Blessed Virgin Mary appeared
three separate times in the year 1830, at the mother-house of the
community at Paris.
The first of these apparitions occurred 18 July, the second 27
November, and the third a short time later. On the second
occasion, Sister Catherine records that the Blessed Virgin appeared
as if standing on a globe, and bearing a globe in her hands, as if
from rings set with precious stones dazzling rays of light were
emitted from her fingers. These, she said, were symbols of the
graces which would be bestowed on all who asked for them.
Sister Catherine adds that around the figure appeared an oval frame
bearing in golden letters the words "O Mary, conceived without sin,
pray for us who have recourse to thee"; on the back appeared the
letter M, surmounted by a cross, with a crossbar beneath it, and
under all the Sacred Hearts of Jesus and Mary, the former
surrounded by a crown of thorns, and the latter pierced by a
sword.
At the second and third of these visions a command was given to
have a medal struck after the model revealed, and a promise of
great graces was made to those who wear it when
blessed. After careful investigation, M. Aladel, the
spiritual director of Sister Catherine, obtained the approval of
Mgr. de Quelen, Archbishop of Paris, and on 30 June, 1832, the
first medals were struck and with their distribution the devotion
spread rapidly.
One of the most remarkable facts recorded in connection with the
Miraculous Medal is the conversion of a Jew, Alphonse Ratisbonne of
Strasburg, who had resisted the appeals of a friend to enter the
Church. M. Ratisbonne consented, somewhat reluctantly, to wear the
medal, and being in Rome, he entered, by chance, the church of
Sant' Andrea delle Fratte and beheld in a vision the Blessed Virgin
exactly as she is represented on the medal; his conversion speedily
followed. This fact has received ecclesiastical sanction, and is
recorded in the office of the feast of the Miraculous Medal.
On 23 July, 1894, Pope Leo XIII, after a careful examination of all
the facts by the Sacred Congregation of Rites, instituted a feast,
with a special Office and Mass, of the Manifestation of the
Immaculate Virgin under the title of the Miraculous Medal, to be
celebrated yearly on 27 November by the Priests of the Congregation
of the Mission, under the rite of a double of the second class. For
ordinaries and religious communities who may ask the privilege of
celebrating the festival, its rank is to be that of a double major
feast. A further decree, dated 7 September, 1894, permits any
priest to say the Mass proper to the feast in any chapel attached
to a house of the Sisters of Charity.
Extracted from The Catholic Encyclopedia