The first feast of the Sacred Heart was celebrated on August 31, 1670, in Rennes, France, through the efforts of Fr. Jean Eudes (1602-1680). From Rennes, the devotion spread, but it took the visions of St. Margaret Mary Alacoque (1647-1690) for the devotion to become universal.
In all of these visions, in which Jesus appeared to St. Margaret Mary, the Sacred Heart of Jesus played a central role. The "great apparition," which took place on June 16, 1675, during the octave of the Feast of Corpus Christi, is the source of the modern Feast of the Sacred Heart. In that vision, Christ asked St. Margaret Mary to request that the Feast of the Sacred Heart be celebrated on the Friday after the octave (or eighth day) of the Feast of Corpus Christi, in reparation for the ingratitude of men for the sacrifice that Christ had made for them. The Sacred Heart of Jesus represents not simply His physical heart but His love for all mankind.
The devotion became quite popular after St. Margaret Mary's death in 1690, but, because the Church initially had doubts about the validity of St. Margaret Mary's visions, it wasn't until 1765 that the feast was celebrated officially in France. Almost 100 years later, in 1856, Pope Pius IX, at the request of the French bishops, extended the feast to the universal Church. It is celebrated on the day requested by our Lord-the Friday after the octave of Corpus Christi, or 19 days after Pentecost Sunday.