Hi ontourUK....May is a beautiful time to visit Corfu with the flowers in full bloom and the green countryside. We visit Sidari usually in mid-to late May and we have been lucky with the weather (in the mid to high twenties). Last year we did have a spectacular thunderstorm with heavy rain but it did not last that long, and remember it makes for a good excuse to dive into a local taverna or bar for an Ouzo or two!! The rain soon clears and gets back to sunny warm weather...unlike Blighty!!
Corfu is beginning to come to life in late May, there will be the odd bar or apartments not open as they often cater for the high season tourists.
Mozzies will be about, but not in their hordes (pesky little blighters). I often say that when we first came to Sidari my 'himself' had his own personal mozzie fan club waiting for him! Just ensure you come armed with the usual potions and lotions (i.e. sprays, liquid mozzie plug ins etc..) and you should be okay. 'Himself' doesn't get bitten now...
I am sure you will love Messonghi in May...You might even be lucky and see the fireflies which are amazing to watch!