It is a Catholic festival celebrating the birth of Saint John the Baptist , which focuses on the mass and procession of Saint John on June 24th, but the festival originates on the summer solstice and was initially about of a pagan festival. People celebrated fertility, associated with the joy of harvests and abundance. Later, similar to what happened with Shrovetide , the Church Christianized this pagan celebration, attributing Saint John the Baptist as its patron saint .
It is not exactly known when the São João do Porto festival began. It is known, from records from the 14th century, since Fernão Lopes , at that time, went to Porto to prepare a visit from the king, having arrived on the eve of São João, he left it written in the Chronicle that it was a day on which in Porto a big party, describing it and how it was experienced by the people of Porto. However, it is very likely that the festival was older, as there was a song from the time that said even the Moiros of Moirama celebrate Saint John .
It was also on Saint John's Day that the Porto City Council met in the Magna Assembly, which would correspond to the current Municipal Assembly, a meeting held in the cloister of the São Domingos Monastery, due to its large space, where the election of councilors and where the most important decisions for the city were made.