The island is at its best in late May, June, and September. In late May, the Girotonno tuna festival adds to the interest. June brings warm weather and sea temperatures that are already good for swimming, without the full summer crowds. September is excellent — the water is at its warmest, the crowds thin, and the light is beautiful.
July and August are peak season. The island fills up, particularly on summer weekends when day-trippers come from the Sardinian mainland. Accommodation is harder to find and more expensive. The ferries run frequently but can have queues for vehicles.
October through April, Carloforte is quiet to the point of sleepiness. Most restaurants and many shops close or reduce hours. The island is still pleasant for walking in October and November, but if you are coming primarily for the sea and restaurants, come in the shoulder season.