Weather in Istanbul actually dictates when to go. If you have your yearly vacation in August, it is set. But if you can schedule your own agenda, then you can pick your dates depending on weather conditions..
Climate is moderate in Istanbul. Weather is suitable for walking nearly 9 months.
Only 2 months in winter are really cold and 1,5 months in summer are really hot. Other than that you are always under a shining sun, without feeling so hot.
The regional location of Istanbul is the second most humid region of the country, with an average humidity of 73%. This makes the hot months really hot.
But there is always the Bosphorus to cool down a bit. Boshorus is windy all year.
The maximum temperature that has been recorded is 41°C(105.8°F).
Best times of the year for Istanbul:
Spring (April, May and first half of June) and Autumn (September, October) are best, because during these months;
Be aware that during the 4-day Kurban Feast and 3-day Ramadan Feast not only banks but nearly all stores (except that the shopping malls and İstiklal Street in Beyoglu of course) are closed.
During the fasting month of Ramazan (Rama-dan) business hours can be chaotic and most of the locals can be in an angry mood, because of the fasting.
And keep in mind that the airports can be overcrowded because of these holidays.
The dates of these holidays are changing every year, so it is wise to check before your trip.
And check the “National Holidays” page for other national holidays.
Remember that during the summer, especially in July and August airplane tickets and hotel prices rise too much and rooms are often overbooked.
Always check beforehand.