Lisbon - On May 13, 1917, three children were guarding sheep in Fatima about 100 kilometres north of the Portuguese capital Lisbon. It was a cloudless day. But suddenly Lucia, Jacinta and Francisco, aged seven to 10, saw something that looked like a flash of lightning.
An incredibly beautiful lady clad in white appeared above an old oak tree.
She was brighter than the sun, with a light more intense than the purest water crossed by the rays of the sun, the children later explained.
On Sunday, 90 years will have passed since that event, which the Vatican recognized in 1930 as an apparition of the Virgin Mary.
Half a million pilgrims are expected at Fatima, one of the most important pilgrimage sites in the Catholic world, which draws more than 3 million visitors annually.
The Virgin appeared to the children five more times from June to October.
The anniversary celebrations will range from the presentation of an encyclopaedia on the apparitions to masses, vigils, exhibitions, conferences, music and theatre, culminating in the inauguration of a new church still under construction.
During the Virgin's last apparition, some 70,000 people are reported to have been present, and many of them saw the sun dance by turning around and flashing colours.
During her apparitions, the Virgin allegedly showed the children a vision of Hell and prophesied the end of World War I and the beginning of World War II as well as the reconversion of Russia to the Christian faith.
She also predicted the early deaths of Francisco and Jacinta, and gave another prophesy which Lucia, who had become a nun, sent to the Vatican in a sealed envelope.
The late Pope John Paul II finally revealed the secret during a visit to Fatima on May 13, 2000.
It was a vision of an attack on a pope, reminiscent of the murder attempt by the Turk Ali Agca on John Paul himself on the significant date of May 13, 1981.
John Paul was convinced that the Virgin of Fatima saved his life.
Lucia died aged 97 in 2005, and Pope Benedict XVI is expected to beatify her like his predecessor did to Jacinta and Francisco.
Benedict XVI is not scheduled to visit Fatima for the main celebrations in October, a decision which has sparked criticism among some Catholics despite his busy agenda.
Fatima is a unique place for believers, even if some complain that it has become too commercial. The majority of the 10,000 residents make a living from tourism.
Sceptics have also claimed that the children actually saw the blonde wife of a British engineer working in the area, and even some theologians have pointed out incoherences in Lucia's testimony.
She lived in "a world of infantile fantasy and constant delirium," said Father Mario de Oliveira, who wrote a critical book on the apparitions in Fatima.
