Hi,
Take a look at this weather website . You can look at previous years weather as well as now.

http://www.wunderground.com/history/station/16641/2007/8/31/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA

For the last 2 years we have visited the area, not actually San Stefanos but quite close, in early May and been very pleased with the weather which was mostly sunny with clear blue skies and very warm temperatures, say 25-28 and certainly hot enough to get burned. Maybe we have been lucky as May is supposed to have a few showers.
It is a lovely month to visit Corfu as the countryside is still green and many wild flowers are around. In the evenings it may be on the cool side (not compared to UK) so a light sweater is suggested for sitting outside.

As the visitor season has only just begun you may find one or two bars not open yet, particularly if you go the 1st week, but it should be no problem and will not spoil your holiday.

Dennis