Originally the month of August had a special feast of the Virgin Mary that was in response to World War II and Fatima.
In the Catholic Church there grew over time specific “devotions” that were assigned to each month of the year. The month of June became associated with the Sacred Heart of Jesus because the feast of the Sacred Heart always falls within that month.
August became known as a month dedicated to the Immaculate Heart of Mary, but currently it is difficult to know why this is the case. There exists no particular feast and the main celebration in August is the Assumption of the Virgin Mary on August 15.
The reason why August became associated with the Immaculate Heart of Mary is because of World War II and the devotion of Pope Pius XII to Our Lady of Fatima.
The world was in turmoil in the 1940s and in 1942, Pope Pius XII responded to the requests of Our Lady of Fatima and consecrated the world to the Immaculate Heart of Mary on October 31, 1942.
When the war continued, Pius XII again turned to the Immaculate Heart.
On May 4, 1944, Pope Pius XII established the feast of the Immaculate Heart of Mary on August 22, the octave day of the feast of the Assumption. He did so that by her intercession may be obtained “peace among nations, freedom for the Church, the conversion of sinners, the love of purity and the practice of virtue.”
This remained the day dedicated to the Immaculate Heart of Mary up until after the Second Vatican Council.
After the revision of the General Calendar, Pope Paul VI decided to switch the feasts of the Immaculate Heart and the Queenship of Mary. The feast of the Immaculate Heart of Mary was joined to the feast of the Sacred Heart of Jesus (celebrated on the Saturday following the feast of the Sacred Heart, typically in June) and the Queenship of Mary was moved to August 22. This was in part to recognize the fact that Mary’s queenship is closely connected to that of her Assumption into heaven.
Even after the switching of the feasts, many Catholics continued to celebrate August as a month dedicated to the Immaculate Heart of Mary, as they felt her message at Fatima needed to be heard in a more extended way.
This designation of August is not an “official” spiritual theme decreed by the Catholic hierarchy, but simply an historical development that grew out of World War II and the message of Our Lady of Fatima.