The Chapel of The Apparitions in Fatima is considered one of the most important Shrines in the World for Catholics, and the second most important shrine in Europe, after the Vatican.
It was here, on this place, that Our Lady appeared to the three little children in 1917, one hundred years ago.
On this exact same place, existed a tree, where Virgin Mary appeared for several times between May and October 1917.

The last apparition was seen by 70.000 people, who witnessed the miracle of the Sun, on the place that today hosts the Sanctuary. The square of the Sanctuary is two times St Peter Square in Vatican. Two of the Children (Jacinta and Francisco), have been canonized in 2017 by Pope Francis on this visit to Fatima, and are now the Youngest Saints (non-martyrs) of the Catholic Church.

 
The Chapel of the Apparitions was built for the first time in 1919 and finalized/opened in 1921, but one year later, 1922, republican people against the church, dinamited the chapel and totally destroyed the building.
By the end of 1922, the new chapel was rebuilt, and today is visited every year by millions of people from all over the world (2017 had 9.4 million people, on opposite to the Vatican that received 10 million visitors).

You can also visit together with the Chapel Of The Apparitions, the following places: Basilica or Our Lady of the Rosary, Most Holy Trinity Basilica, Candle Procession/Lighting, Houses of the Children, Museum of Fatima, Berlin Wall, and much more.
Fatima is located only 1h20m from Lisbon, and the most recommended way to visit is by car or doing a private tour.

You can see here a suggestion for a Private Tour of 7h, departing daily from Lisbon and returning back to Lisbon: https://lisbonprivatetours.com/fatima-shrinesanctuary-full-day-private-tour-from-lisbon-driver-guide/