The apparition at Fatima is one of the great apparitions of the Catholic Church. Our Lady revealed certain truths to three children in Portugal.
From Hilary White:
“To prevent this, I shall come to ask for the Consecration of Russia to My Immaculate Heart.” This must be done by the pope in conjunction with all the bishops of the world before 1960. If this were done, Russia would be converted and a period of peace would be given to the world. If Her requests were not granted, “Russia will spread its errors throughout the world, raising up wars and persecutions of the Church. The good will be martyred, the Holy Father will have much to suffer and various nations will be annihilated.”
But, really, who can say it better than Archbishop Fulton Sheen? I was going to read the Wiki entry which I heard isn’t bad as a basic introduction, but I think listening to the Venerable Bishop Fulton Sheen is a better option than me trying to write 500+ words on the subject.
Our Lady of Fatima, Ora Pro Nobis.