May to October is the practical window for visiting Linosa. July and August are warmest — sea temperatures reach 26°C — but also the period when the handful of guesthouses fill quickly and the small ferry is most crowded. June and September are better: still warm, less congested, and prices are lower.
June to August is the nesting season for Caretta caretta sea turtles. Females come ashore at night to lay eggs on the beaches, particularly Cala Pozzolana di Ponente. If you are staying overnight, you have a genuine chance of witnessing this. Locals know the patterns and will sometimes alert visitors; approach quietly and avoid torchlight.
Outside the main season, ferry connections become infrequent and some of the small guesthouses close. The island is habitable year-round for the 400 or so residents, but winter visits require planning.