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.

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

Here is a link to forecasts for Istanbul