Sanırım cevap için geç kaldım :) Yaz aylarında daha rahat seyahat edebilirsin.

From the middle of March to June and September to November are highest seasons(Istanbul & Cappadocia). If you're thinkin of summer holidays then i'll recommend you.. June and late August to early September.

It probably depends on where in Turkey you plan to visit. I visited Istanbul in March, but people often seemed surprised to see a foreigner at such an off time of year. And with good reason, as it rained and poured nearly 3/4 of the time I was there. I'd suggest a summer month, for sure!