Cappadocia is best in late spring (May, early June) and autumn (September, October), when daytime temperatures are warm enough for walking and the air is clear for ballooning. July and August are very hot — into the 30s Celsius — and crowded, especially in Göreme and Üchisar.
Winter brings a different and very photogenic Cappadocia: the fairy chimneys frosted with snow, the valleys silent, almost no other tourists. Hot-air balloons fly less frequently and need particularly settled weather, but if you catch a clear morning the snow-and-tuff colour palette is unforgettable. Nights are cold, well below zero in mid-winter; pack accordingly.