In Turkey there is a noticeable variety of climates, with considerable differences between the areas, and also with some microclimates due to the exposure of slopes and coasts.
The coastal areas have generally a mild climate, but the coast of the Black Sea is colder than that of the Mediterranean. The interior has a continental climate, with cold and snowy winters, and hot and dry summers, though nights remain usually cool.
Summer is dry and sunny throughout the country, except on the north-east coast. In all the inland and south-central regions, the driest months are July, August and September.
The map below shows the climate zones of Turkey.

When To Go

Almost the whole of Turkey, except the north-eastern coast, enjoys a dry and sunny summer, but it can be hot, even on the coast. So in general it is preferable to visit the country in spring and autumn: already in April along the west coast (see Izmir); in May, June and September in Istanbul; in May, June and September in the Anatolian Plateau and Ankara (with a preference for the month of May, and remembering that nights can be a bit cold); in April, May and the first half of October along the southern coast and in the semi-desert areas of the south-east; in June and September in the eastern highlands; in July and August in high mountains.

For a beach holiday, along the north and north-east coast the best months are July and August, but remember that east of Istanbul, summer is not very sunny, and can be rainy, especially in the areas most exposed to the westerlies (while bays such as that of Samsun are relatively sheltered). In the easternmost part of the coast, it goes even worse and the rain may be abundant even in summer.
Along the west coast, the bathing season runs from June to September, the temperature is high and the sun shines. However, as we have seen, the sea is slightly cooler, and sometimes the Meltem, the northern wind, blows.
Along the southern coast, summer is very hot, so you may prefer the months of June and September, to avoid the period of the most intense heat, which beyond a certain level, can be felt even at the beach.

What To Pack

In winter, on the north and west coast (Izmir and Trabzon), and in the south on the border with Syria, warm clothes, sweater, jacket, raincoat or umbrella. In Istanbul, warm clothes, down jacket, scarf and hat for cold spells; in Antalya and the south coast, spring/autumn clothes, sweater, jacket, raincoat or umbrella. In Ankara and the plateau, very warm clothes, down jacket, hat, scarf, gloves.
In summer: on the west and south coast (Izmir, Antalya), light clothes, light sweatshirt for the evening, scarf for the breeze; on the north coast (Istanbul, Trabzon), you can bring a sweatshirt or sweater for safety; in the eastern part of the Black Sea coast, a raincoat or umbrella. In Ankara and the plateau, light clothes for the day, sun hat, a sweatshirt or sweater for the evening; in the eastern part of the plateau, you can add a jacket for the evening. In the south, on the border with Syria, lightweight clothing, of natural fibres, desert turban, possibly a sweatshirt for overnight stays outdoors.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
To visit mosques, you must have your shoulders and legs covered, and bare feet.
For women, it is best to avoid shorts and miniskirts in rural and non-tourist areas.