Some 400,000 pilgrims from around the world will welcome the Argentine pontiff on the giant esplanade that faces the Sanctuary of Our Lady of Fatima as he arrives in his "Popemobile", while countless others will follow proceedings on television.
The Virgin is said to have appeared six times in Fatima, north of Lisbon, between May and October 1917 to three impoverished, barely-literate children -- Jacinta, 7, Francisco, 9, and their cousin Lucia, 10.
She apparently shared three major prophesies with the trio at a time marked by the ravages of the First World War and Church persecution in a relatively new Portuguese republic.
According to interpretations of what Lucia revealed much later on, the first secret gave a vision of hell, while the second warned of a second devastating war and the rise of communist Russia.
The third secret, which Lucia kept to herself for years, is believed to have been a prediction of the 1981 assassination attempt on Pope John Paul II.
His successor Benedict XVI, however, later said she had foreseen the "suffering" of the Church, which at the time was racked by pedophilia scandals.
To believe or not to believe?
As such, the 12,000-strong town has become a major draw for pilgrims, with millions visiting every year.
On Friday and Saturday, some 40,000 walkers, 2,000 journalists, 2,000 priests, 71 bishops, eight cardinals, 350 patients hoping for a miracle and hundreds of volunteers will be present, according to Carmo Rodeia, one of the sanctuary managers.
Pilgrims will journey from countries as varied as South Korea, Japan and China, Mexico and Colombia.
Nationals from Portugal -- where 89 percent of the 10.3 million inhabitants are Catholic -- will be out in force, prompting the Pope to speak in Portuguese.
On Saturday -- the 100th anniversary of the Virgin Mary's first reported apparition -- Pope Francis will canonise Jacinta and Francisco who were apparently responsible for two miracles.
Talk of apparent apparitions and miracles outside of those described in the Old and New Testaments does not sit comfortably with everyone.
But the Church is nevertheless very attentive to popular piety -- forms of prayer and worship inspired by believers' culture and experiences rather than by official religious teachings.
"The Church must base itself on what people have experienced," says theology professor Ermenegildo Manicardi.
"If it didn't, it would have an absolutist position."
For instance, while the Vatican has yet to recognise the reported continuous apparitions of the Virgin Mary in the Bosnian town of Medjugorje since 1981, it has dispatched a special envoy to examine "the needs" of the millions of faithful who go there.
Donated bullet to shrine
Fatima's apparitions, though, have been officially recognised by the Catholic church since 1930.
And it's not just regular faithful who are attracted to the site, but popes too.
With Francis included, four pontiffs will have visited the Marian sanctuary in half a century.
Arguably the most devoted was the late Pope John Paul II, who made three pilgrimages to the shrine located 130 kilometres (81 miles) north of Lisbon.
He attributed his narrow escape from death following an assassination attempt at St. Peter's Square on May 13, 1981 -- the anniversary of the reported apparitions -- to the intervention of the Virgin Mary.
He later donated the bullet extracted from his abdomen to the Fatima shrine, a move described by Manicardi as "an extreme gesture of popular piety".
Pope Francis is also sensitive to the tradition of popular piety in his native Latin America.
"John Paul II and Francis are both pastoral popes who go to the people," says Manicardi, explaining their attraction for popular piety, unlike Pope Benedict XVI who was more of a theologian.
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