The feast commemorates a vision of the Virgin Mary in Fatima, Portugal where three children saw the Blessed Mother on May 13, 1917.

our_lady_fatimaThe apparitions of Mary to the children began on May 13 and on this day the Catholic Church usually celebrates the feast of Our Lady of Fatima.
 
At Fatima, Mary revealed to the children three "secrets". The first two were published by the only one of the three children to survive into adulthood, Lucia, who became a Carmelite nun and died at the age of 97 in 2005. The first secret related to a vision of Hell and the second to how to save people from ending up in Hell. The third secret was published in 2000, by order of Pope John Paul II and related to the murder of a Pope and other religious leaders. When the secret was published many commentators linked the content to the attempted assassination of John Paul II in 1981.
 
On his way to Portugal in 2010, Pope Benedict answered questions posed by Father Frederico Lombardi, Vatican spokesperson.

"Before all else, I want to say how happy I am to be going to Fatima, to pray before Our Lady of Fatima. For us, Fatima is a sign of the presence of faith, of the fact that it is precisely from the little ones that faith gains new strength, one which is not limited to the little ones but has a message for the entire world and touches history here and now, and sheds light on this history. In 2000, in my presentation, I said that an apparition – a supernatural impulse which does not come purely from a person’s imagination but really from the Virgin Mary, from the supernatural... Beyond this great vision of the suffering of the Pope, which we can in the first place refer to Pope John Paul II, an indication is given of realities involving the future of the Church, which are gradually taking shape and becoming evident. So it is true that, in addition to moment indicated in the vision, there is mention of, there is seen, the need for a passion of the Church, which naturally is reflected in the person of the Pope, yet the Pope stands for the Church and thus it is sufferings of the Church that are announced.

The Lord told us that the Church would constantly be suffering, in different ways, until the end of the world. The important thing is that the message, the response of Fatima, in substance is not directed to particular devotions, but precisely to the fundamental response, that is, to ongoing conversion, penance, prayer, and the three theological virtues: faith, hope and charity. Thus we see here the true, fundamental response which the Church must give – which we, every one of us, must give in this situation. As for the new things which we can find in this message today, there is also the fact that attacks on the Pope and the Church come not only from without, but the sufferings of the Church come precisely from within the Church, from the sin existing within the Church. This too is something that we have always known, but today we are seeing it in a really terrifying way: that the greatest persecution of the Church comes not from her enemies without, but arises from sin within the Church, and that the Church thus has a deep need to relearn penance, to accept purification, to learn forgiveness on the one hand, but also the need for justice. Forgiveness does not replace justice. In a word, we need to relearn precisely this essential: conversion, prayer, penance and the theological virtues. This is our response, we are realists in expecting that evil always attacks, attacks from within and without, yet that the forces of good are also ever present and that, in the end, the Lord is more powerful than evil and Our Lady is for us the visible, motherly guarantee of God’s goodness, which is always the last word in history."