You can live 4 seasons in Turkey. In winter, you have lots of opportunities to have a wonderful holiday doing skiing, snowboarding etc. or in summer you can have a fantastic, hot holiday at a beach or at a pool bar drinking a cold drink in the sun. In spring or fall, there are warm, sometimes rainy days here in Turkey.  Bring a cardigan and your umbrella.

July and August finds hot (40C+) temperatures quite common along the popular southern and western coasts.  In addition to the heat, the Mediterranean coast is also quite humid.  The lowlands along the Syrian border are extremely uncomfortable in July and August.

April/May and September/October are generally great times to visit with diminshed crowds, typically wonderful weather and, in the fall, warm seas. 

Here is a link to an overview of Istanbul's Weather

Here is a link to forecasts for Istanbul