"The heart has always been seen as the 'center' or essence a person ('the heart of the matter,' 'you are my heart,' 'take it to heart,' etc.) and the wellspring of our emotional lives and love ('you break my heart,' 'my heart sings,' etc.). Devotion to the Sacred Heart of Jesus is devotion to Jesus Christ Himself, but in the particular ways of meditating on His interior life and on His threefold love – His divine love, His burning love that fed His human will, and His sensible love that affects His interior life," Pope Pius XII wrote in his 1956 encyclical "Haurietis Aquas" ("On Devotion To The Sacred Heart").
The Friday following the second Sunday after Pentecost is the Feast of the Sacred Heart. On the Feast of the Sacred Heart, we can gain a plenary indulgence by making an Act of Reparation to the Sacred Heart.
From the earliest days of the Church, writes the Catholic Encyclopedia, "Christ's open side and the mystery of blood and water were meditated upon, and the Church was beheld issuing from the side of Jesus, as Eve came forth from the side of Adam. It is in the 11th and 12th centuries that we find the first unmistakable indications of devotion to the Sacred Heart. Through the wound in the side, the wound in the Heart was gradually reached, and the wound in the Heart symbolized the wound of love."
This general devotion arose first in Benedictine and Cistercian monasteries of that time, but specific devotions became popularized when St. Margaret Mary Alacoque (1647-1690), a Visitation nun, had a personal revelation involving a series of visions of Christ as she prayed before the Blessed Sacrament. She wrote, "He disclosed to me the marvels of His Love and the inexplicable secrets of His Sacred Heart." Christ emphasized to her His love – and His woundedness caused by man's indifference to this love.
— Catholic News Agency