La Pescheria is the open-air fish market that fills the square and the surrounding alleys just behind Piazza del Duomo, accessed through a vaulted archway next to the cathedral. It runs every morning except Sunday, starting around six and winding down by midday, and it is one of the great living spectacles of Mediterranean daily life.
Swordfish are laid out whole, sea urchins are split with a flick, tuna is hacked into great red slabs, and the whole thing is loud, theatrical and entirely indifferent to spectators. The surrounding streets have trattorias and bars that buy from the market in the morning and serve it at lunch. Even if you are not buying anything, walk through.