|Sacred Heart, Michael D. O'Brien|
Today the Church celebrates the Solemnity of the Sacred Heart of Jesus. The feast is held the Friday after the Second Sunday after Pentecost. We can trace the origin of the devotion back to the Middle Ages.
This devotion already existed in the Middle Ages as a private devotion; it appears as a liturgical feast in 1675 following the apparitions of Our Lord to Saint Margaret Mary Alacoque. In these revelations the saint was given to know with special depth the need to make reparation for her personal sins and for the sins of the whole world, and to respond to Christ's love. Our Lord asked her to extend the practice of frequent Communion, especially on the first Friday of every month, in a spirit of reparation. He asked for the first Friday after the octave of the Most Holy Sacrament to be dedicated to a special feast to glorify his Heart. The feast was celebrated for the first time on the 21 June 1686. Pius IX extended it to the whole Church. In 1928 Pius XI gave it the splendour it has today (In Conversation with God, Vol. 6. p. 315).
[W]hen we talk about the heart of Jesus, we stress the certainty of God's love and the truth of his commitment to us. When we recommend devotion to the sacred heart, we are recommending that we should give our whole self to Jesus, to the whole Jesus — our soul, our feelings and thoughts, our words and actions, our joys. That is what true devotion to the heart of Jesus means (Christ is Passing By, 164)For a great overview of this solemnity, I suggest you watch the following video from the Apostleship of Prayer apostolate.