At Fatima on 13 July 1917 the shepherd children, Lucia, Francisco and Jacinta, were shown the vision of hell by Our Lady, who then said, so kindly and sadly:

“You have seen hell where the souls of poor sinners go. To save them, God wishes to establish in the world devotion to my Immaculate Heart. If what I say to you is done, many souls will be saved and there will be peace ...  I shall come to ask for the consecration of Russia to my Immaculate Heart, and the Communion of Reparation on the First Saturdays ... In the end, my Immaculate Heart will triumph. The Holy Father will consecrate Russia to me and she will be converted, and a period of peace will be granted to the world."
On 10th December 1925, the Blessed Virgin appeared again to Lucia, with the Child Jesus. Mary rested her hand on Lucia’s shoulder, and showed her a heart encircled by thorns in her other hand. The Child Jesus spoke first:

"Have pity on the Heart of your Most Holy Mother. It is covered with the thorns with which ungrateful men pierce it at every moment, and there is no one to make an act of reparation to remove them."  Then Mary said:
"My daughter, look at My Heart surrounded with thorns with which ungrateful men pierce it at every moment by their blasphemies and ingratitude. You, at least, try to console me, and say that I promise to assist at the hour of death, with all the graces necessary for salvation, all those who, on the first Saturday of five consecutive months, shall confess, receive Holy Communion, recite five decades of the Rosary and keep me company for fifteen minutes while meditating on the mysteries of the Rosary, with the intention of making reparation to me."

It signifies the intimate union of Mary’s Immaculate Heart with the Sacred Heart of her Son. Mary’s heart, “opened with the words, ‘Woman, behold your son’ (Jn 19: 26) is spiritually united with the heart of her Son, opened by the soldier’s spear”, Pope John Paul II taught in his homily at Fatima, on 13th May 1982.

In this devotion, Jesus Himself desires reparation for the sins which pierce His Mother’s sorrowful heart, maternal symbol of her merciful love and desire for the salvation of all mankind in Him. If you practice this devotion, Mary promises to come with the “graces necessary for salvation” at the supreme moment of death.