The summer season runs from late May through September. July and especially August are hot and crowded — most of Greece holidays here, and accommodation prices triple. June and September are the best months: warm sea, long days, manageable crowds.
Winter is mild and very quiet; most resort hotels close from November to April, but the inland villages, the Petralona Cave, and the archaeological sites stay open year round. Mount Athos is open to permit-holders year round, but the boats from Ouranoupoli run a reduced schedule from November to March and may be cancelled in rough weather.