Quick Answer
Late May to June and September to mid-October. You get warm swimming water (24-26°C), sunshine, and 30-50% fewer tourists than July-August. Peak season (July-August) brings 35°C+ heat, sold-out ferries, and doubled accommodation prices.
Island by Island
Santorini: Best May-June and September-October. July-August is cruise ship madness — up to 18,000 cruise passengers daily in Fira/Oia. Mykonos: Best June and September. July-August is party peak (if that's your thing). Crete: Longest season — April to November. Big enough to escape crowds. Rhodes/Kos: Best May-October. Warmer than western islands. Cyclades (Naxos, Paros, Milos): Best June-September. Less hyped than Santorini/Mykonos, better value.
Shoulder Season Sweet Spot
Late September is the real sweet spot. Sea temperature is at its warmest (26°C after months of summer heating), crowds vanish after schools restart, and prices drop 30-40%. The only downside: some beach bars and clubs on party islands close by mid-September. Early October is still warm enough for swimming on most islands.
What to Avoid
Easter week: domestic tourism spikes — ferries and flights book out. August 15 (Assumption Day): Greece's biggest holiday — everything is packed and prices peak. Meltemi winds: strong northerly winds hit the Cyclades June-September, especially July-August. They cool you down but can delay ferries and make beaches on the north side of islands unusable.
Money-Saving Tips
Inter-island ferries: book on FerryHopper.com — Blue Star Ferries is the most reliable. Flying between islands: Sky Express runs small planes between major islands. Stay on lesser-known islands: Milos, Naxos, Folegandros, and Sifnos offer comparable beauty to Santorini at half the price. Eat at tavernas with paper tablecloths and locals inside — the food is better and cheaper than harbour-front restaurants.