Above image colorized by Deacon Chris Mazzacano

"I do not promise you happiness in this world, but in the next."

Our Lady of Lourdes to Saint Bernadette

On February 11, 1858, fourteen-year old Bernadette Soubirous had an encounter with the Blessed Virgin Mary at the place of a cave called Massabielle in Lourdes, France:
 

…I came back towards the grotto and started taking off my stockings. I had hardly taken off the first stocking when I heard a sound like a gust of wind. Then I turned my head towards the meadow. I saw the trees quite still: I went on taking off my stockings. I heard the same sound again. As I raised my head to look at the grotto, I saw a lady dressed in white, wearing a white dress, a blue girdle and a yellow rose on each foot, the same color as the chain of her rosary; the beads of the rosary were white....From the niche, or rather the dark alcove behind it, came a dazzling light…

In spite of the skepticism (and punishment) of her famliy, she returned to the cave, and on eighteen separate occasions encountered the "lady dressed in white", who would utilize the diminutive peasant girl to enter into the Tradition of the Church the devotion of First Saturdays (in reparation for blasphemy against the Blessed Virgin) and to validate in  aspecial way the dogma of "Mary, the Immaculate Conception", promulgated by Pope Pius IX four years earlier in 1854.  

Bernadette described the "Lady" as a "tiny maiden" no more than 12 years old, and of roughly the same size as her 4'7" frame (Bernadette would later be disappointed that artistic renderings would reflect a taller, older woman instead).

Although there were only a few words exchanged between the Blessed Mother and Saint Bernadette, there are two which emblemize the purpose of the miraculous apparition:
 

"I do not promise you happiness in this world, but in the next."

(February 18, 1858)
 

"I am the Immaculate Conception."
(March 25, 1858)
 

The first quote is a great exhortation to the perseverance of Christians through difficulty and suffering, but the second quote, "I am the Immaculate Conception" was an earth shattering confirmation of Pope Pius IX's declaration of the dogma of the Immaculate Conception, issued four years earlier.  

It is also of note that Mary appeared as a 12-year old girl, which, tradition has conjectured, was the approximate age of the Virgin when she became pregnant with Our Lord (and culturally appropriate for her time).  The connection of the Incarnation with her Immaculate Conception speaks to the dogma's necessity.  Indeed her womb was the Ark of the New Covenant -- a pristine vessel in which to carry a perfect Son.  

Further, Our Lady's assuming of the title of "The Immaculate Conception" served to authenticate her appearances when it was vigorously investigated by the Church.  Indeed, the Bishop of Tarbes observed with great amazement that a young, illiterate girl cut off from the outside world would even utter the words "the Immaculate Conception" in any context.  It simply beggared belief that she would have an inkling of this dogma, given her circumstances.  Much of what took place at the Vatican was a mystery to most Catholics before the 20th century and the eventual Information Age.  

On January 18, 1862, the Bishop of Tarbes Betrand Severt Laurence declared the following regarding the alleged Marian apparitions:

"We are inspired by the Commission comprising wise, holy, learned and experienced priests who questioned the child, studied the facts, examined everything and weighed all the evidence. We have also called on science, and we remain convinced that the Apparitions are supernatural and divine, and that by consequence, what Soubirous saw was the Most Blessed Virgin. Our convictions are based on the testimony of Soubirous, but above all on the things that have happened, things which can be nothing other than divine intervention."

Source: https://en.wikipedia.org/wiki/Our_Lady_of_Lourdes