I was over on Zante last year may 23rd-30th and the weather was pretty rubbish if i'm honest, we had rain at some point every single day, at times torrential, but when it wasn't raining it was really nice, not too hot and not too windy, it was nothing like the same week usually is on Rhodes where I live, when the last week in may is usually cloudless and reaches the high 20's or low 30's daily with very little chance of rain even if you want some.
Went to Zante the same week in 1997 and it was absolutely ferocious, 38 degrees all day everyday, much like july or august, so I guess really in the Ionian it's far less predictable year on year than elsewhere in Greece.