As best sailing period regarding the weather we recommend May till July. The weather conditions are quite stabilized, days are long, the water is warm enough for swimming and ports and bays will not be overcrowded.
From middle of August on you may meet some thunderstorms and depressions. Normally this goes for one month (can be in September or October) and after this month the weather can be nice till December. (No guarantees). We recommend the side from the University of Athens for all information about wind and weather in greek seas.
Another good and easy to use side for worldwide weather information is