The Three Hill Altar on the "Saddle" used for celebrating the mass for the pilgrims.
Csíksomlyó became a pilgrimage site in 1567, when Hungarian king John II Sigismund Zápolya wanted to convert the Székely population of the upper Csík to Protestantism. The Székelys refused to abandon the Catholic faith and resisted. A battle took place on a nearby field, on Saturday before Pentecost 1567, from which the Székelys emerged victorious. The monks saw this as a sign of the care of Virgin Mary, and since then, this event has been commemorated by a pilgrimage when the believers gather on Pentecost every year. Beside its religious importance, the pilgrimage has also become a community event demonstrating spiritual unity of Hungarian people living in and outside the historical region of Transylvania.

Csíksomlyó hosts the largest annual pilgrimage in Central Europe. Regardless of their religious affiliation, three to four hundred thousand Hungarians gather to this southeastern fringe of the Carpathian Basin on every Pentecost Saturday in order to pay a visit to a five-hundred year old wooden statue of the chief patron of Hungarians and the protector of Szeklers, Holy Mary.