April to June and September to October are the best months to visit Forio. The weather is warm, the sea swimmable, the crowds manageable, and the prices lower than in summer. La Mortella and Giardini Ravino are open and at their best in spring when the gardens are flowering. The Poseidon thermal gardens operate at reduced capacity and significantly reduced prices outside July and August.
July and August are high season across the island. Forio is busier than the rest of Ischia at this time because of the beaches at Citara and Cava dell'Isola. Citara in particular gets crowded in August; arrive by 9am for a spot on the free sections or expect to pay for a lido chair. Prices for everything — ferries, restaurants, places to stay — peak in mid-August. If you're coming in August, book early and accept the trade-offs.
The thermal springs at Sorgeto Bay are open year-round and free every day of the year. Winter visits are possible; Forio is quieter but the island doesn't close entirely. Many restaurants and gardens shut from November to March, but the old town, the churches, and the springs remain. A January evening at Sorgeto, with no one else there and the cold air on your face and the hot water at your feet, is something quite specific.