The Festa de São João is Porto's most important annual celebration, held on the night of June 23rd. The entire historic centre turns into a street party from dusk until dawn: grilled sardines served from temporary stalls in every alley, wine and beer flowing freely, bands playing in the squares, and the characteristic ritual of hitting strangers on the head with plastic hammers, stuffed leeks, or sprigs of garlic. Fireworks over the Douro conclude the official programme around midnight, but the party continues until morning.
The celebration has its roots in the midsummer festivals that predated Christianity, adapted into the feast of St. John the Baptist. Porto's version is the largest and most exuberant in Portugal, and the whole city participates — this is not a tourist event but a genuine local tradition that happens to welcome visitors. If you are in Porto on or around June 23rd, arrange your schedule around the São João. Hotels and restaurants fill up; book accommodation early and expect the city to be running on reduced sleep for two or three days around the date.