Devotion to the Sacred Heart of Jesus has as its dogmatic foundation the Incarnation of the Second Person of the Blessed Trinity. On account of the hypostatic union, every part of our Lord's Human Nature is worthy of adoration. Hence, therefore, we adore His bodily Heart, beating in His Bosom. We alo honor the Heart of Jesus as a reminder, or symbol, of His love for us, and we are moved to make Him a return of love, because He has loved us and He is not loved by men. Love, consecration, and reparation are the characteristic acts of this devotion. In this form it is now solemnly approved by the Church.
On the feast of the Sacred Heart celebrated on the Friday after the Octave of Corpus Christi an act of reparation is prescribed for recitation in every church in the world. On the feast of Our Lord Jesus Christ, King, the last Sunday of October, an act of consecration of the human race is prescribed. Though this devotion was practiced by saintly souls before 1675, it is due to the apparitions of our Lord to Saint Margaret Mary Alacoque in the Visitation Monastery at Paray-le-Monial that the feast of the Sacred Heart is now kept on the day assigned by Our Lord. In spite of much opposition the feast was allowed in 1765, and extended to the world in 1856; in 1929 it was raised to the highest rank. Special manifestations of the devotion are the Communion of Reparation on the first Friday of the month, and the Holy Hour in union with Our Lord in His Agony. [Source]