The best time to sail in Greece and Turkey is between late May and late Oct.
Earlier in the season is possible, but those glorious sunny-all-day Med conditions are not guaranteed; we can still get some dodgy weather early in May. The Turkish season tends to extend a little later than Greece; it stays warm and calm longer, the “lights-off” time for most tourist businesses in Turkey is Oct 30. In Greece, conditions can be a little unpredictable late in Oct (but it can still be a great time to sail, for those who hate crowds!)
Winds – the typical meltemi pattern of summer winds tends to follow the temperature. It blows the strongest in mid-summer, and is calmer in early and late season. In some parts of Greece (notably the Ionian), it’s a nice, predictable afternoon breeze; anywhere mid-Aegean or further east, forget any such reliability – the meltemi will blow when it feels like it!