Denizli sits inland in the Aegean uplands, which makes its climate different from the coast. Summers are hot and dry — daytime temperatures above 35°C are common in July and August — and the white travertines at Pamukkale become blinding under a midday sun. Winters are cool, frequently wet, and the surrounding mountains see snow.
The best months are April–May and September–October, when temperatures are pleasant for walking the ruins and the surrounding hills are at their greenest (or in autumn, their golden best). Spring brings wildflowers across the plateau and a lower volume of tour buses than the peak July–August window.
If you only have one day for Pamukkale, plan to be on the terraces either early in the morning or in the last two hours before sunset — both for the lower temperatures and for the way the low sun turns the travertines pink and gold.