It takes place on the third Tuesday after Easter, coinciding with the municipal holiday.

During the weekend that precedes this day, religious festivities and processions are already celebrated in the place.

This Pilgrimage is the most popular festival in the city and takes place in the large area next to the chapel of Nª Srª de Mércoles. The origin of this festival has to do with a vote of thanks to Our Lady for having freed the city from the plague and other plagues. The cult dates back to the 16th century. The Parish of São Miguel da Sé and the Sanctuary of Our Lady of Mércoles Commission are responsible for organizing it.

Source: https://lifecooler.com/artigo/atividades/romaria-de-nossa-senhora-de-mrcoles/321511