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)