The weather can be unpredictable out of season but,ironically it is best to book two weeks not one at this time of year. One week can be a washout and the next glorious but you rarely get two bad weeks in a row.
Once the weather settles - from late May to mid-September - it is almost guaranteed to be hot, bright and clear. Too hot sometimes, particularly on southern coasts of southern islands when tourist police can clear beaches at midday to prevent daft sunbathers getting grilled alive. Average sea level temperatures are 27° C in July.

In winter low-pressure systems move in from the Atlantic, bringing rain while cold winds blow in from the Balkans. The same low-pressure systems draw warmer winds from the south so there can be a marked difference in winter temperatures between north and south islands. The Ionian Islands and the western mountains get most of the rain, often three times that in the sheltered east. The Ionian and Aegean islands are notably in October and November.