Turkey is such a big land with very volatile weather conditions therefore the best time to visit Turkey is up to destination you are going and the aim of your travel. If you can't hold on too much sun and heat it is not recommended to visit especially Aegean and Mediterranean coasts in Summer ( June, July, August ) while the temperatures are above 40 C. average.( in this case; May, September, October recommended ). If you are looking for a round trip in Turkey to see the historical sites ( May and September are the best months to go for) . Winters are quiet warm and rainy by the seasides though inner land and Eastern Anatolia temperatures can go - 30 to -40 Degrees