The Virgin Mary appeared to three children in Fatima, Portugal in 1917. She appeared one time each month starting on May 13 and ending on October 13. Mary identified herself as Our Lady of the Rosary.

Some believe that the Fatima apparitions truly began in April — perhaps on April 13 — because sometime before the children saw Our Lady, they were first visited by an angel to prepare them.

The three children were named Lucia, Francisco and Jacinta. During her apparitions, Our Lady gave them messages for mankind and secrets about the future. Francisco and Jacinta died as children but Lucia lived until she was an old woman.
At one point, Our Lady showed the children a vision of Hell. “You have seen Hell, where the souls of poor sinners go,” she told them. “To save future souls God wishes to establish in the world the devotion to My Immaculate Heart. If people do what I tell you, many souls will be saved…If my requests are granted, Russia will be converted and there will be peace. If not, she will scatter her errors throughout the world, provoking wars and persecution of the Church. The good will be martyed, the Holy Father will have much to suffer, and various nations will be destroyed…But in the end, my Immaculate Heart will triumph. The Holy Father will consecrate Russia to me, Russia will be converted, and a certain period of peace will be granted to the world”.
Our Lady also said, “When you recite the Rosary, say at the end of each decade: Oh My Jesus, forgive us our sins, save us from the fires of hell, and lead all souls to Heaven, especially those in most need of Your Mercy.”
Our Lady promised to give a sign on the final day of her apparitions. On October 13, 1917, over 70,000 gathered to be present at the final apparition. The Miracle of the Sun occurred and was witnessed by all. Here is an eyewitness account from an atheist reporter:
“Before the astonished eyes of the crowd, whose aspect was biblical as they stood bare-headed, eagerly searching the sky, the sun trembled, made sudden incredible movements outside all cosmic laws – the sun ‘danced’ according to the typical expression of the people.”
And this is the testimony of an eye doctor who was there:
“The sun, at one moment surrounded with scarlet flame, at another aureoled in yellow and deep purple, seemed to be in an exceeding fast and whirling movement, at times appearing to be loosened from the sky and to be approaching the earth, strongly radiating heat.”
The secrets seemed to be very accurate although some is left up to interpretation:
The first secret was a vision of Hell:
Our Lady showed us a great sea of fire which seemed to be under the earth. Plunged in this fire were demons and souls in human form, like transparent burning embers, all blackened or burnished bronze, floating about in the conflagration, now raised into the air by the flames that issued from within themselves together with great clouds of smoke, now falling back on every side like sparks in a huge fire, without weight or equilibrium, and amid shrieks and groans of pain and despair, which horrified us and made us tremble with fear. The demons could be distinguished by their terrifying and repulsive likeness to frightful and unknown animals, all black and transparent. This vision lasted but an instant. How can we ever be grateful enough to our kind heavenly Mother, who had already prepared us by promising, in the first Apparition, to take us to heaven. Otherwise, I think we would have died of fear and terror.
You have seen hell where the souls of poor sinners go. To save them, God wishes to establish in the world devotion to my Immaculate Heart. If what I say to you is done, many souls will be saved and there will be peace. The war is going to end: but if people do not cease offending God, a worse one will break out during the Pontificate of Pope Pius XI. When you see a night illumined by an unknown light, know that this is the great sign given you by God that he is about to punish the world for its crimes, by means of war, famine, and persecutions of the Church and of the Holy Father. To prevent this, I shall come to ask for the Consecration of Russia to my Immaculate Heart, and the Communion of reparation on the First Saturdays. If my requests are heeded, Russia will be converted, and there will be peace; if not, she will spread her errors throughout the world, causing wars and persecutions of the Church. The good will be martyred; the Holy Father will have much to suffer; various nations will be annihilated. In the end, my Immaculate Heart will triumph. The Holy Father will consecrate Russia to me, and she shall be converted, and a period of peace will be granted to the world.