What was confided on this subject in 1917, is as follows:

She asked for them to be taken to Heaven, and the most holy Virgin answered:

— Yes. I will take Jacinta and Francisco soon. But you are to stay here some time longer. Jesus wishes to make use of you to make me known and loved. He wants to establish in the world devotion to my Immaculate Heart. I promise salvation to those who embrace it, and these souls will be loved by God, like flowers placed by me to adorn His throne.

— …My Immaculate Heart will be your refuge and the way that will lead you to God.

Letter of Lucia to Aparicio (1927-12-??)
Sister Lucia's Memoirs. [pdf]
Appendix I

On December 10, 1925, the Most Holy Virgin appeared to her, and by Her side, elevated on a luminous cloud, was the Child Jesus. The Most Holy Virgin rested Her hand on her shoulder, and as She did so, She showed her a heart encircled by thorns, which She was holding in Her other hand.

At the same time, the Child said:

— Have compassion on the Heart of your most holy Mother, covered with 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 the Most Holy Virgin said:

— Look, my daughter, at my Heart, surrounded with thorns with which ungrateful men pierce me 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 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 fifteen mysteries of the Rosary, with the intention of making reparation to me.

Letter of Lucia to Aparicio (1927-12-??)
Sister Lucia's Memoirs. [pdf]
Appendix I, Text of the great promise of the Heart of Mary

— It is true that the Mother Superior alone can do nothing, but with My grace, she can do anything. It is enough that your confessor gives you permission, and that your superior announce this for it to be believed by the people, even if they do not know who it was revealed to.

— But my confessor said in his letter that this devotion already exists in the world, because many souls receive You every first Saturday of the month, in honour of Our Lady and the fifteen mysteries of the Rosary.

— It is true, My daughter, that many souls begin, but few persevere to the very end, and those who persevere do it to receive the graces promised. The souls who make the five First Saturdays with fervour and to make reparation to the Heart of your Heavenly Mother, please Me more than those who make fifteen, but are lukewarm and indifferent.

— My Jesus! Many souls find it difficult to confess on Saturday. Will You allow a confession within eight days to be valid?

— Yes. It can even be made later on, provided that the souls are in the state of grace when they receive Me on the first Saturday, and that they had the intention of making reparation to the Sacred Heart of Mary.

— My Jesus! And those who forget to form this intention?

— They can form it at the next confession, taking advantage of their first opportunity to go to confession.