Pope Francis is embarking on a high-profile pilgrimage to Fatima in central Portugal, a shrine where he plans to make two shepherd children saints.
It is 100 years since the two - and a third child - reported seeing the Virgin Mary while tending sheep. The third is also on the way to sainthood.
Portugal has boosted security and reimposed border controls, suspending the Schengen open borders pact. More than a million pilgrims are expected.
The three revered children are dead.
The Roman Catholic Church attaches great value to their visions, as Mary is believed to have revealed truths to help mankind. The Church says the first vision came on 13 May, 1917.
In a video message to the people of Portugal, the Pope said he was going to present himself to Mary "and I need to feel you close, physically and spiritually, so that we are one heart and one mind".
Jacinta and Francisco Marto are to be canonised on Saturday. They died in the 1918-1919 European influenza pandemic.
The so-called three secrets of Fatima were written down by the third child - their cousin Lucia dos Santos - who died in 2005, aged 97. The beatification process for her began in 2008.
What are the three secrets?
They are prophecies written down by Lucia, years after the apparitions that the three said they had witnessed. She spent her adult life as a nun at a convent in Coimbra.
The first two secrets in Lucia's account were revealed in 1942.
The first described a terrifying vision of hell, with a "great sea of fire", demons and human souls
The second is interpreted as Mary's prediction that World War One would end and that World War Two would start during the papacy of Pius XI
Mary also called for the "consecration" of Russia, saying: "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"
Lucia sealed the third secret in an envelope, which was handed to the Vatican in 1957 and only revealed in 2000
It described an angel demanding "penance!", then the Pope and other clergy climbing a mountain, only to be killed by soldiers firing bullets and arrows.
What does the Vatican say about them?
According to Pope Francis's predecessor, Benedict XVI, the visions described in the three secrets are "meant to mobilise the forces of change in the right direction".
They are not like the Bible - a text he describes as a "public revelation", which ended "with the fulfilment of the mystery of Christ as enunciated in the New Testament".
The Fatima visions are "private revelations", he writes. Their purpose is "to help live more fully" in accordance with Christ's teaching.
The late Pope John Paul II was shot by a Turkish gunman on 13 May 1981.
He believed that his survival was due to Mary's divine intervention, and that the third secret had predicted the attack on him.
ohn Paul donated the bullet to Fatima, and it was inserted into the crown adorning a statue of Mary there.
What about Pope Francis's visit?
He follows John Paul II and Benedict XVI, who also made pilgrimages to the Fatima Sanctuary. He is to arrive on Friday afternoon, for a 24-hour visit.
There will be a torch-lit procession on Friday evening and a Mass on Saturday.
Portugal is deploying 6,000 police and emergency workers at the site daily. Concrete blocks have been placed on approach roads, to stop any terrorist "ramming" attack with a vehicle.
Only nine border crossings are open, with systematic checks, as Portugal has suspended Schengen.
A Portuguese man called Carlos Gil is known as "rent-a-pilgrim", AFP news agency reports. He charges €2,500 (£2,110; $2,717) to walk to Fatima and worship there on behalf of a Catholic who cannot make the trip.
Local accommodation is far more expensive than usual, as hotels and residents cash in on the papal visit.
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