The First apparition of Our Lady occurred on Sunday May 13th 1917 as the children were pasturing their flock as usual at the Cova da Iria, which was about a mile from their homes.
They were playing when suddenly a bright shaft of light pierced the air. They described it as a flash of lightning. It was not really lightning, but rather the reflection of a light that approached little by little. Frightened by the flash, the children looked around at the sky that was clear and bright without the least spot of a cloud. No breeze stirred, the sun was strong, and there was no hint anywhere of a storm that might be responsible for a flash of lightning. The children, however, thought that they had better head home in case it might start raining.
As they descended the hill, another flash of lightning took them by surprise. Panicky with fear, they took a few steps and looked towards the right. There, standing over the foliage of a small holm oak, a lady dressed all in white, more brilliant then the sun, shedding rays of light, clear and stronger than a crystal glass filled with the most sparkling water, pierced by the burning rays of the sun.
The lady spoke to them and said: “Fear not! I will not harm you.”
“Where are you from?” the children asked.
“I am from heaven,” the beautiful lady replied, gently raising Her hand towards the distant horizon.
“What do you want of me?” Lucia asked.
“I came to ask you to come here for six consecutive months, on the thirteenth day, at this same hour. I will tell you later who I am and what I want. And I shall return here again a seventh time.”
Lucia said : “Do you come from heaven…and will I go to heaven?”
“Yes, you’ll go.”
“Him too, but he will have to say many rosaries”. In the end Our Lady asked: “Do you wish to offer yourselves to God, to endure all the suffering that He may please to send you, as an act of reparation for the sins by which He is offended, and to ask for the conversion of sinners?”
“Yes, we do.” said the children.
“You will have to suffer a lot, but the grace of God will be your comfort.”
Then She opened her hands with a loving gesture of a mother who offers Her heart. From it an intense light departed that seemed to go through them. The vision vanished telling them: “Recite the rosary every day to obtain the peace for the world and the end of the war.”
And She disappeared.
(Taken from http://www.theholyrosary.org/fatimaapparitions)