Depending on whether the goal of your trip is beachfront leisure, explorations into antiquity, high-octane pursuits, or coordinating your vacation with the Tulip Festival or music festival performances in the Aspendos Theatre (in which case, see the Calendar of Events), the seasonal ebbs and flows of tourism follow some general patterns. If you're hoping to hit the village resorts and historical sites of the Mediterranean or Aegean, the absolute best time to go is during the "shoulder season" months of April, May, mid- to late September, and October, when families send their kids back to school, museum sites are less crowded, and the heat is more bearable, particularly when the midday sun reflects and magnifies off of the white stone and marble of archaeological sites. For watersports enthusiasts (rafting, canoeing), the spring melt requires nothing but the most expert or at least courageous. For that matter, Cappadocia is a great destination for rafting in the spring as well as for the autumn colors, while hiking, biking, and camping around the coastal villages are great spring or fall diversions.
In the winter, the coastal towns shut down like a submarine before a descent, although things tend to miraculously reopen (at high-season pricing) for holidays, including Christmas and New Year's. Cappadocia takes on an otherworldly wonderland aspect covered with a dusting of snow, but icy conditions may ruin a horseback-riding trek. And while the dreary days of December and January are inhospitable for a visit to the southern coast, these months, although often rainy, almost never see a line at a museum in Istanbul. Also, the hilltops of the Gallipoli Peninsula can get very wet and windy, so a pilgrimage to the battlegrounds -- if not coinciding with Anzac Day -- may be best planned for the summertime.