The physical heart, symbol of the spiritual heart
The Fathers of the Church consider that when, from the Cross, Our Lord Jesus made the Blessed Lady the mother of St. John, He also appointed her the mother of all men.
Thus, Mary’s heart is the physical symbol of her boundless love for God and human kind. But Our Lady’s physical heart is also the symbol of her spiritual heart. Thus, in the Immaculate Heart of Mary we also honor her inner life, her virtues, her perfect purity, her boundless humility, her affections and her sorrow.
Fatima and the Immaculate Heart of Mary - The Five First Saturdays Devotion
The Blessed Mother appeared alongside the Child Jesus who stood over a luminous cloud. Our Lady rested one hand on the Child’s shoulder while she held on the other hand a heart pierced with thorns.
The Child said, "Have pity on the Heart of your Most Holy Mother which is covered with thorns with which ungrateful men pierce it at every moment with no one to extract them with an act of reparation."
Our Lady expressed her request in the following words,
"See, my daughter, My Heart surrounded with thorns with which ungrateful men pierce me at every moment with blasphemies and ingratitude. You, at least, make sure to console me and announce that all those who for five months, on the first Saturdays, go to Confession, receive Communion, say five decades of the Rosary and keep me company for 15 minutes meditating on the mysteries of the Rosary, with the purpose of making reparation to Me, I promise to assist them at the hour of death with all the graces necessary for the salvation of their souls."
Act of Consecration to the Immaculate Heart of Mary