Centro Storico (Duomo) in March

April through May and September through October hit the sweet spot. Weather's mild, crowds are manageable, and restaurants have outdoor seating without the summer sweat. July and August turn Centro Storico into a human oven — the marble reflects heat, there's zero shade, and every tourist in Europe seems to converge on the Duomo steps. December brings Christmas markets to Piazza Santa Croce, but also rain that makes those cobblestones slippery as ice. Early morning visits work year-round. Hit the Duomo complex at 8am opening, and you'll have decent photos before the tour groups arrive. Evening strolls after 7pm let you see the neighborhood as locals do, when the day-trippers head back to their cruise ships.