Pamukkale is open year-round; but the answer to "when" is really tangled up with "which hour of the day."
Seasons
Spring and autumn are the most balanced: the weather is ideal for walking, the terraces aren't too crowded, and the light is soft for photos. In summer, the sun is strong, so I recommend visiting early in the morning or in the late afternoon — you'll enjoy cooler air, a more relaxed pace, and even better light; in winter, the cold air naturally makes it cold there too — but Pamukkale has its own beauty in every season, and misty mornings bring an atmosphere all their own.
Time of day
The biggest difference isn't the season — it's the hour. Tour buses usually arrive around midday. I build my tours around either the first entry in the morning or the late afternoon — fewer crowds and softer light both.
Send me your dates and we'll pick the best window for that specific day together.