The feast of the Immaculate Heart of Mary is celebrated on the Saturday of the third week after Pentecost, the day after the Solemnity of the Sacred Heart of Jesus. The Gospel reveals the Heart of Mary because it shows how Mary sought and followed Jesus: purity of heart is to follow Jesus!
Mary did not understand everything instantly, but she meditated on what she did not understand, with an open heart, waiting to understand better: her heart was humble, docile, and open to an ever deeper revelation, to an ever stronger light.
The corresponding devotion is based on the Marian theology of St. Bernard, the private revelations to St. Gertrude and St. Mechtilde, and the visions of St. Margaret Mary Alacoque in the 17th century, and was initiated by St. John Eudes in Autun in 1648.
The Augustinians obtained this feast from the Holy See for their order in 1807 (at the time celebrated on the Sunday of the octave of the Assumption). In 1855, the Congregation of Rites approved the celebration of a Mass on this occasion. In 1880, Pope Leo XIII extended it to the diocese of Rome.
In 1917, Mary said to the children of Fatima: "You have seen hell, where the souls of poor sinners go. In order to save them, God wants to establish devotion to my Immaculate Heart. If you do what I tell you, many souls will be saved and you will have peace." She also said, "In the end my Immaculate Heart will triumph!" (July 13, 1917).