Not that i've been in September and a lot of the 'regulars' on this forum obviously have a better idea, but when we decided to visit this year (coming in June), i also thought about September, but i'm a bit of a weather geek and i checked a weather website called wunderground.
This site has historical data going back about 20 years for anywhere on earth! Corfu history shows high humidity and most years in September, thunderstorms, so that's why we chose 2nd half of June for our visit. Watch! We'll prob have thunderstorms in June!
Can't afford too many 'non-sunshine' days or i'll get earache from the better half for choosing the wrong weeks!!