On October 31, 1942, on the day of the solemn closing of the Jubilee of the Apparitions of Fatima in Portugal, Pope Pius XII, speaking on the radio, consecrated the world to the Immaculate Heart of Mary, to answer the call of our heavenly Mother. He renewed this important act on December 8, 1942.
In 1944, in the midst of World War II, the Holy Father consecrated the entire human race to the Immaculate Heart of Mary, placing it under her all-powerful protection.
At the same ceremony he decreed that the whole Church should celebrate a feast each year in honor of the Immaculate Heart of Mary, to seek "the peace of nations, the freedom of the Church, the conversion of sinners, the love of purity, and the practice of virtues" through her intercession.
He established this feast on August 22, in the octave of the feast of the Assumption of Mary.
Source: Free