The best time to visit Istanbul is during the spring, which lasts from April to the middle of June. Temperatures are comfortable, with highs in the low 20s, and sunshine is abundant. Summers are sunny too, with temperatures in the upper 20s. The humidity can be oppressive, though. It's also the peak tourist season, so prepare to fight the crowds.
Fall is also a great time to visit. Conditions are similar to spring, though the wet season starts in October. Winters are cool, wet and dreary. The sun only makes rare appearances. Snowfall is fairly common, but doesn't last long as temperatures rarely dip below freezing for long.