Istanbul has four seasons. Although with the global climate change, these days spring and fall are becoming less noticeable.
Spring is the finest time in the city. It is during March, April and May. April and May is usually really nice, mild and sunny for the most part, although evenings tend to get chillier. June, July and August is summer and from mid- July till end of August, it can get quiet humid and warm. Many istanbulites leave the city in August to visit the beaches.
Fall is lived in September, October and November. These are also quiet nice months when the humidity drops and one can enjoy being outdoors in mild but not too warm weather. From mid- November onwards, it starts getting chillier. And finally December, January and February is winter. It rains during the winter and the temperature drops down to zero celsius and Istanbul gets snow every year. Sometimes the snow is just a couple of days, sometimes it lasts for a few weeks.
Late spring (April & May) and early fall (September & October) are the nicest months of the year in terms of the weather.