In the early Middle Ages, devotion to the Heart of Jesus was connected with devotion to his Passion—his Heart being the symbol of his redemptive love. Though the devotion was known for centuries, it only became widespread in France through the zealous preaching and admirable writings of St. John Eudes. It was, however, first to St. Margaret Mary Alacoque that Christ revealed the treasures of his Sacred Heart and asked that a feast be celebrated in its honor. When these revelations became public, the devotion became popular, for it now took on specific devotional practices, such as that of the First Fridays and reparation to the Sacred Heart. St. Claude La Colombière worked with St. Margaret Mary in trying to have such a feast approved in the Church. It was in 1765 that Pope Clement XIII first allowed such a feast to be celebrated in Poland and at the Archconfraternity of the Sacred Heart in Rome. Pope Pius IX then ordered (1856) it to be celebrated throughout the world on the Friday following the Second Sunday after Pentecost, when it is still celebrated. In 1899, Pope Leo XIII consecrated the human race to the Sacred Heart of Jesus.

~Joseph N. Tylenda, Saints and Feasts of the Liturgical Year (Washington, DC: Georgetown University Press, 2003), 117.