Pope Francis is to visit a town in Portugal to honour two poor shepherd children whose visions of the Virgin Mary 100 years ago marked one of the most important events of the 20th century Catholic Church.
Francis will arrive in the shrine town of Fatima to celebrate the centenary of the apparitions and canonise the children.
He is hoping the message of peace that they reported 100 years ago, when Europe was in the throes of World War One, will resonate today.
Francis's deputy, Cardial Pietro Parolin, said the importance of Fatima lies specifically in the fact that poor, illiterate children were able to convey a powerful message of love and forgiveness at a time of war when "the talk was of hatred, vendetta, hostilities".
Thousands of pilgrims, waving flags from as far as Venezuela, Argentina and Cuba, braved a steady cold rain on Friday as they waited for Francis, many spending the night outdoors to make sure they had a good spot.
Over the past several days, church groups, families and individuals have made their way to Fatima, about 150 kilometres (90 miles) north of Lisbon, some completing the final leg of the pilgrimage on their knees in prayer.
Carrying candles, rosaries and roses, they have made their way to the statue dedicated to Our Lady of Fatima or tossed wax body parts - ears, hearts, limbs - into a huge fire to pray for healing.
"For me it is the second time I am here with a pope, first with John Paul II and now with Papa Francisco," said pilgrim Elisabete Fradique Conceicao in between rain showers.
"They are simple men and that simplicity makes sense when you think what happened here 100 years ago."
On May 13 1917, while they were grazing their sheep, the children saw the first of half a dozen visions of the Virgin Mary.
They said she revealed to them three "secrets" - apocalyptic messages foreshadowing the Second World War, hell, the rise and fall of communism and the death of a pope - and urged them to pray for peace and turn away from sin.
At first doubted by the local Catholic Church and even their parents, the children's story gained believers and was eventually accepted as an authentic apparition by the church in 1930.
The children being canonised, brother and sister Francisco and Jacinta Marto, who were nine and seven at the time of the apparitions, died of influenza two years later.
Their cousin, Lucia dos Santos, at 10 the ringleader of the group and who became the main raconteur of their tale, is on track for beatification, the first step towards becoming a saint.
Her case could not begin until after her death in 2005.
Fatima has long been associated with St John Paul II, given the Polish-born pope credited the Virgin Mary with having saved his life in 1981 when a would-be assassin shot him on Fatima's feast day - May 13 - in St. Peter's Square.
John Paul made the first of his three pilgrimages to Fatima the following May, and one of the bullets fired at him now adorns the crown of the Madonna at the shrine.
Like John Paul, the Argentinian-born Francis is exceedingly devoted to the Madonna, thanks in large part to the strong role that Marian devotions play in the popular piety of Latin American Catholics.
Before every trip he takes, Francis brings a bouquet of flowers to an icon of Mary at the Rome basilica dedicated to her name, St Mary Major.
On his first trip as pope, to Brazil, he prayed at the shrine to the Madonna at Aparecida.
He has done the same at the shrine of Our Lady of Guadalupe in Mexico, at the shrine of Caacupe in Paraguay and the Virgin of the Charity of Cobre in Cuba.
One major shrine he has avoided visiting is in Medjugorje, Bosnia-Herzegovina, where six youngsters reported having had regular, repeated visions of the Virgin starting in 1981.
A Vatican-appointed doctrinal commission finished its study years ago, but Francis has not released the results.
He has cautioned against such tales of regular visions, however, and has said Marian devotion must be to "the real Madonna. Not the Madonna who's like the head of a post office who every day sends a different letter saying 'My children, do this and then the next day says do that'."
The church says the Fatima case is altogether different.
In a video message on the eve of his departure, Francis urged all faithful to join him, physically or spiritually, in Fatima.
"With all of us forming one heart and soul, I will then entrust you to Our Lady, asking her to whisper to each one of you: 'My Immaculate Heart will be your refuge and the path that leads you to God.'"