Hi Pimo,
We usually go for the last two weeks in August. The weather is hot, and it is usually pretty windy. When the wind drops, it does tend to be very hot. I went to Plakias in mid-July in 2012, and actually found it much hotter than August, partly because there was less wind. I wouldn't rely on that though, as it might have been a particularly hot July.
We go to the beach early or late and stay in the shade as much as possible at the hottest time. We never use air-conditioning, and have generally found it reasonably okay to sleep, even in August.
Given the choice, I'd rather go in June or September, but we are limited to school holidays at present.
Enjoy it whenever you go - we will be there the last two weeks in August this year, so perhaps see you then?
Arnaki