The best time to visit Turkey has to be Spring (April to June) for many reasons such as winter is over, just before the influx of all other tourists eg. Asian tourists in June, US tourists begins in July, Europeans in August etc Depending on which part of Turkey you are planning to visit, there are some months you do not want to go eg. Black Sea region is not fun before mid-April due to the rain and of course, if you are not going to Istanbul, you will not want to visit other regions in the winter months unless you plan on doing some winter sport like skiing. Months that most tourists should take note are the fasting month of Ramadan (Islamic calender follows the lunar calender so it is not the same every year, it is wise to check before your trip, for 2006, it begins 24th Sep.) and during their public holidays especially Kurban Bayrami (aka Eid el Adha) is usually a 4 to 5 day holiday (sometimes a whole week) and again, have to check before your trip due to the different dates each year, can be anywhere from mid-Nov to the first few days of Jan depending on the year). Also the summer months of July through to September are their vacation months. If you plan to travel during these months, please make sure you have confirmation on all your arrangement especially train or plane tickets and hotels as well as some extra cash as banks will be closed (unless you plan to use the ATM). Most of the religious holidays will see the closed of some bazaars and attractions so be prepared with contingencies. The warmest month in Istanbul is August usually around 28 celsius with April in the mid-teens and beginning June in the twenties. Rainfall is heaviest usually in December and lowest usually in August. So go and enjoy Turkey !