The best time to visit Turkey is spring (March to mid-June) when the weather is moderate and the days are long. Do note that you may get a day or two of rain, particularly in April. We visited at the end of May and had a day’s washout. The other days, however, were glorious. Autumn (Sep-Oct) is the second-best time to visit. Summer is hot and crowded while winter can be cold and rainy.
In Cappadocia and more mountainous areas summer can be unbearably hot while winter can see snowfall! Autumn (mid September to October) is still a good option, but the days are shorter and there’s more chance of rain.