The veneration of the Immaculate Heart of Mary is in a very close connection with the devotion to the Sacred Heart of Jesus. The Oratorian Priest John Eudes, besides celebrating the Sacred Heart with his religious community in 1646, he also introduced a celebration of the Heart of Mary. It took until the beginning of the 19th cent. for this devotion to gain an official recognition under Pius VII and it became an obligatory celebration for the whole Church in 1944 (Pius XII), under the influence of the apparitions in Fatima (1917).
First, it was celebrated on August 22 (Octave of the Assumption of the Blessed Virgin), but then it was transferred to Saturday after Sacred Heart of Jesus (third Saturday after Easter).