We're thinking about 2-3 weeks in Turkey next year. I don't like crowds, and I don't like relentless hot weather. I typically prefer Mediterranean destinations in the spring and fall. I'm thinking that mid- to late-September might be the sweet spot for Turkey (maybe arrive around Sept 10, stay through the last few days of September).
I assume the worst of the crowds start to dissipate soon after August rolls over into September, and by mid-September things are starting to get quiet, but not quite off-season dead, and most things will still be open and going. Weather should be warm (warm enough to swim) but not oh-my-god hot, water temps still nice and warm for comfy swimming, with the stored heat from the dwindling summer.
Of course, I've never been there, so I'm just making guesses. For those who know Turkey, how close are my guesses? Any other downsides to traveling in mid-late September?