Here's a short excerpt from my book on the rosary:

A Mystery in History: Fatima and the Miracle of the Sun

In 1917, in Fatima, Portugal, the pure faith of three very young children prompted an apparition of the Mother of God. At the time, Jacinta, Francisco, and Lucia were seven, nine, and ten years old, respectively. After praying the rosary with the innocent, childlike faith that Jesus praised (Matthew 18:3), Mother Mary appeared to them and referred to herself as Our Lady of the Rosary. She would appear to them five more times, encouraging them to pray the rosary for an end to World War I. But when the children told the people of their village what they had seen, people doubted and scoffed. So the Blessed Mother promised a miracle that would convince the people that the children were telling the truth.
On October 13th, 1917, over 70,000 people showed up to see if there was really going to be a miracle. They were not disappointed. The sun moved in the sky, changed colors, and then seemed to move closer to earth. The rain-soaked ground was immediately dried. Eye witnesses said the sun was easy to look at without squinting, and some said they saw an image of the Virgin Mary. The whole crowd fell to its knees, many of the people were in tears. Many sick or infirmed people were healed, and many doubters were convinced.
It is interesting to notice that in the book of Revelation, Mary is described as the woman “clothed with the sun” (Revelation 12:1). In 1930, the Church confirmed that this was, in fact, a miracle, and both Francesco and Jacinta have been canonized as saints. There is much more to the visitation of the Blessed Virgin Mary to the children of Fatima, but for the moment it’s important that Mary encouraged the praying of the rosary, and affirmed its power.