To answer this question, you need to first decide what kind of trip you're taking. Are you going for the food? To see historic sights? To shop?
If you're coming for the food: The best time for a culinary trip is between April and October. In winter, Istanbul can be cold, rainy, and windswept — which isn't ideal for eating out on the streets. Still, it does mean that most restaurants offer their own take on borscht (beet soup) or other hearty stews.
If you want to visit Istanbul's many historic sights: The summer months (June through August) are an excellent time to visit if you want to avoid crowds at museums and mosques. In winter, however, museums are closed on Mondays and Tuesdays so if you want to see all the sights in one day, aim for Thursday through Sunday. The best time for visiting historic sites is during spring or autumn when temperatures are milder.
If you plan on shopping: Shopping-wise, May through September is your best bet because all of Turkey's major trade fairs take place during these months — meaning lots of discounts!
Generally, The best time to visit Istanbul is from April to June or September to October. These are the hottest months of the year in Istanbul but also the wettest, and the weather is cooler and drier during the spring and autumn months.
The city's temperature varies from a high of 38 C in summer. It can drop below zero degrees Celsius (32 degrees Fahrenheit) at night during December through February, so bring warm clothing even if it is sunny during the day.
In conclusion, The best month to visit Istanbul is April. The weather is temperate, with an average temperature of 15 degrees Celsius (60 F) and only a slight chance of rain.