Mediterranean, weather in Greece is nice, especially in mid-season (spring and the beginning of autumn): about 25°C (77°F), and sun almost every day. In summer, it's very hot, and temperature can rise to 40°C (104°F). Such a heat is unbearable in Athens. In the Cycladic island, even if it's so hot, there is the meltemi, summer wind from the north. But if you have the sun in your room during the day, you won't have a very nice night...
Don't wait for rain in july and august. The rainiest months are in winter. In march-april and october-november, you can have some rain (not every year), but in general it's very sunny. But if it rains, take hot clothes for the night (in spring), because in this case, the nights are not very hot. And if it's not very sunny, you will take cold showers: reservoirs are on the roofs and heating by the sun.
Last, don't forget there are mountains in Greece: if you go there, of course, it's colder...