The many miracles which have been performed through the intercession of the Blessed Virgin at Lourdes prompted the Church to institute a special commemorative feast, the "Apparition of the Immaculate Virgin Mary." The Office gives the historical background. Four years after the promulgation of the dogma of the Immaculate Conception (1854), the Blessed Virgin appeared a number of times to a very poor and holy girl named Bernadette. The actual spot was in a grotto on the bank of the Gave River near Lourdes.

The Immaculate Conception had a youthful appearance and was clothed in a pure white gown and mantle, with an azure blue girdle. A golden rose adorned each of her bare feet. On her first apparition, February 11, 1858, the Blessed Virgin bade the girl make the sign of the Cross piously and say the rosary with her. Bernadette saw her take the rosary that was hanging from her arms into her hands. This was repeated in subsequent apparitions.  

With childlike simplicity Bernadette once sprinkled holy water on the vision, fearing that it was a deception of the evil spirit; but the Blessed Virgin smiled pleasantly, and her face became even more lovely. The third time Mary appeared she invited the girl to come to the grotto daily for two weeks. Now she frequently spoke to Bernadette. On one occasion she ordered her to tell the ecclesiastical authorities to build a church on the spot and to organize processions. Bernadette also was told to drink and wash at the spring still hidden under the sand.

Finally on the feast of the Annunciation, the beautiful Lady announced her name, "I am the Immaculate Conception."

The report of cures occurring at the grotto spread quickly and the more it spread, the greater the number of Christians who visited the hallowed place. The publicity given these miraculous events on the one hand and the seeming sincerity and innocence of the girl on the other made it necessary for the bishop of Tarbes to institute a judicial inquiry. Four years later he declared the apparitions to be supernatural and permitted the public veneration of the Immaculate Conception in the grotto. Soon a chapel was erected, and since that time countless pilgrims come every year to Lourdes to fulfill promises or to beg graces.

http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2015-02-11