Most tourists visit between April and the end of October. July and August are the busiest months (and the warmest). April through June, and September and October offer more temperate weather, smaller crowds, and somewhat lower hotel prices.
Istanbul tends to be hot in summer, cold in winter. The Mediterrranean and Aegean coasts have mild winters and hot summers; you can swim along either coast from late April into October. The Black Sea coast is mild and damp, with a rainfall of 90 inches per year. Central and easter Anatolia can be extremely cold in winter, and its roads and mountaind passes closed by snow; summers bring hot, dry weather, with cool evenings.