The month of August is given the special honor of being devoted to the devotion of the Immaculate Heart of Mary. While we have been unable to find a definitive statement explaining this, a couple of guesses can be made.
First, the Feast of the Assumption of Mary occurs on the 15th of August. Don't forget to go to Mass!
Second, the Feast of St. John Eudes, considered to be the father of the devotion of the Immaculate Heart by Pope Pius X, is celebrated on August 19th.
Third, before Vatican II the Feast of the Immaculate Heart used to be celebrated on August 22nd.
Before looking closely at the actual devotion it is good to look back at where the focus on the heart of Mary we should take a look at where Mary's heart is mentioned in Scripture. There are two mentions of Mary's heart in the Gospel of Luke. First, “But Mary kept all these words, pondering them in her heart.” (Luke 2:19) Second, “And he went down with them, and came to Nazareth, and was subject to them. And his mother kept all these words in her heart.” (Luke 2:51)
In his Encyclical, Redemtor hominis, Blessed John Paul II says that “We can say that the mystery of the Redemption took shape beneath the heart of the Virgin of Nazareth when she pronounced her “fiat”.
St. Faustina saw Mary's heart as a channel of graces: “I saw, between heaven and earth, the Mother of God, clothed in a bright robe. She was praying with her hands folded over her bosom, her eyes fixed on heaven. From her heart issued forth fiery rays, some of which were turned towards heaven while others covered our country.” Diary of St. Faustina.
So Mary's heart is seen in these examples as the source of her original “Yes” to God and the graces for mankind that come from her Son.
Devotion to her Immaculate Heart seems to have begun in the twelfth century based on the writings of the saints. From the Catholic Encyclopedia:
It is only in the twelfth, or towards the end of the eleventh century, that slight indications of a regular devotion are perceived in a sermon by St. Bernard (Deduodecim stellis), from which an extract has been taken by the Church and used in the Offices of the Compassion and of the Seven Dolours. Stronger evidences are discernible in the pious meditations on the Ave Maria and the Salve Regina, usually attributed either to St. Anselm of Lucca (d. 1080) or St. Bernard; and also in the large book “Delaudibus B. Mariae Virginis” (Douai, 1625) by Richard de Saint-Laurent. Penitentiary of Rouen in the thirteenth century. In St. Mechtilde (d. 1298) and St. Gertrude (d. 1302) the devotion had two earnest adherents. A little earlier it had been included by St. Thomas Becket in the devotion to the joys and sorrows of Mary, by Blessed Hermann (d.1245), one of the first spiritual children of St. Dominic, in his other devotions to Mary, and somewhat later it appeared in St. Bridget's “Book of Revelations”. Tauler (d. 1361) beholds in Mary the model of a mystical, just as St. Ambrose perceived in her the model of a virginal soul. St. Bernardine of Siena (d.1444) was more absorbed in the contemplation of the virginal heart, and it is from him that the Church has borrowed the lessons of the Second Nocturn for the feast of the Heart of Mary. St. Francis de Sales speaks of the perfections of this heart, the model of love for God, and dedicated to it his “Theotimus“.
Even though there are many earlier references to the devotion, the devotion to the Immaculate Heart wasn't regularized and promoted enough to later become a practice throughout the Church until the 1600's by St. John Eudes in France. His book The Admirable Heart of Mary is an in-depth explanation of the devotion. During his beatification Pope Pius X declared him the Father of Devotion to the Immaculate Heart.
In the visions of Fatima on July 13th, 1917, Mary told the children “to save poor sinners, God wishes to establish in the world devotion to my Immaculate Heart.” In a series of visions in 1925 and 1926, the oldest of the visionaries, Lucia Dos Santos, received a further message from Mary asking that the five first Saturdays be devoted to help make amends for the offenses committed against her heart by the blasphemies and ingratitude of men.
In 1942, as the war Mary prophesied raged through Europe and the Church celebrated the 25th anniversary of the original visions at Fatima, Pope Pius XII consecrated the Church and the whole world to Mary's Immaculate Heart. Also in 1942 he established the Feast of the Immaculate Heart to fall on August 22 during the Octave of the Assumption.
On May 4th, 1944 Pope Pius XII extended the celebration of the Feast of the Immaculate Heart of Mary to the universal Church. Following Vatican II the feast day was moved to the Saturday following the Feast of the Sacred Heart of Jesus.
More Resources about the Immaculate Heart of Mary: