Hottest months are usually July and August. Daytime high temperatures range from 29-38 degrees Centigrade (82-95F), a "normal" day would be in the middle of this range. By the sea there is, of course, usually a breeze which moderates how this feels. Though it may be cooler in the mountains, especially at night, it can feel hotter in places sheltered from the sea breeze than at lower altitudes. Ultimate months for swimming, cooling down and using plentiful strong sunscreen.
June and September normally deliver fine sunny days and  moderate-to-hot temperatures. Water is warm enough for swimming and best in September. Day time peaks between 23 and 29 degrees.
May - take an average of our comments for the months before and after! It is "normally" warm and sunny, much or most of the month.
March, April, October and early November often provide warm and very enjoyable days - it gets a lot less predictable at these edge of season months, but you'll find it un-crowded, naturally Corfu and for some the most enjoyable. We recommend a warm outdoor jacket and favourite sweater for the odd chilly day and for evenings. Day time peak temperatures range from 17 to 26 degrees.
December - often good, but it is winter, so warm outerwear and waterproof aids are essential - same for January and February. During these first two months of the year there "should" be some rain, but again, not every day. Winter (December to February)  often feels cold - the island is equipped for summer. Of course if you call Novosibirsk home, Corfu might seem mild in the winter. The wind and sea combine on an island to produce some dramatic weather conditions at times. Day time peak temperatures range from 9 to 18 degrees (even colder in the mountains, warmer on a few days if you are lucky).
Everywhere we've been for the last ten years we hear "it never used to be so hot / cold / dry / wet at this time of year" - weather worldwide is in a changing mode (this happens every 100 years approximately), so if you are wise you'll bring clothes to cater for the range of weather that may occur and take the idea of seasonal temperatures seriously - then you can only be pleasantly surprised!

Best advice, always bring a set of outer warm and wind-defeating clothes, just in case. We receive many questions about what the weather will be like in a particular month. One thing we realize: the weather is changeable from year to year, especially in the "shoulder" months April to June and October to November.
The only remedy is to bring (heavier luggage!) a variety of clothing on your trips to Corfu - or - be happy to shop locally!
Wish it were sunny where you are? It's not sunny every day in Corfu, in the winter it does rain. Sometimes the rain seems like an eternal force, driving or dribbling, driven by occasionally ferocious winds. However, there are some sunny days and a few partly sunny days in winter - it really is a matter of luck. Summer - ranges from very warm and sunny to very hot and sunny! One of our number was swimming daily in the sea quite happily until November 15th the other year and he hates cold water!