A century ago, three shepherd children claimed to have visions of the Virgin Mary.
The two girls and a boy said she told them three secrets, which were later interpreted as foretelling the Second World War, the rise and fall of Communism and the death of a pope.
Pope Francis is due to arrive Friday in their Portuguese hometown of Fatima. He will canonize two of them, while the third child is also on her way to becoming a saint.
Up to a million people are expected to attend the canonization Mass on Saturday.
Their visions marked one of the most important events in 20th century Catholicism and have become known as the "Three Secrets of Fatima."
For days, pilgrims carrying candles, rosaries and roses have made their way to the statue dedicated to Our Lady of Fatima, with some completing the last part on their knees.
Portugal has boosted security ahead of the pontiff's visit and has reinstated border controls restricting freedom of movement from other European states.
In a video message to the people of Portugal before his journey, Pope Francis said he would "present" himself to the Virgin Mary.
Visions of the Madonna are very important in the Catholic faith and the story of Fatima is universally known among Catholics.
What are the 'Three Secrets of Fatima'?
While grazing their sheep on May 13, 1917, Francisco and Jacinta Marto and their cousin Lucia dos Santos said they saw the first of several visions of the Virgin Mary who had three messages for the world.
Two years later, both of the Martos siblings died of influenza leaving Lucia to share the secret messages with the world.
At first, the children were not believed. But their visions were accepted as authentic apparitions by the Church in 1930.
According to the prophecies written down by Lucia, the first vision the children saw was one of hell and came with a call to prayer and conversion, which if not heeded would lead to another world war.
The second was that Russia would "spread her errors" throughout the world if people did not turn to God.
It wasn't until 2000 that the Vatican disclosed the long-awaited third secret, describing it as a prediction of the 1981 assassination attempt of Pope John Paul II.
John Paul II had credited the Virgin Mary with saving his life and one of the bullets fired at him was placed in the crown of a statue of her in Fatima.
Lucia died in 2005 and is currently on the track for beatification, the first step towards becoming a saint. However, the process could not start until after her death.