Officials and media reported a steady stream of pilgrims arriving on foot or by car at the small farming town 110 kilometers (about 70 miles) north of Lisbon.
Authorities expected around 250,000 people would attend outdoor services at the shrine over the weekend.
The main event is a candlelight mass at midnight on Saturday. May 13 is the anniversary of the first apparition.
Many pilgrims walk to Fatima from all over the country to pay penance or carry out a promise made to the Virgin Mary, the AP reports.
This year's celebrations coincide with the 25th anniversary of an attempt on the life of Pope John Paul II by a Turkish gunman in St. Peter's Square in 1981.
The pontiff, who died last year, credited the Virgin of Fatima with saving his life after he was shot.
Like the shrine at Lourdes, France, Fatima draws huge numbers of visitors - local officials estimate 6 million - each year.