Spring (March to May) and autumn (September to November) are the best times to visit Ouarzazate. Temperatures are comfortable — warm enough for desert excursions without the crushing heat of summer — and the light on the kasbahs and gorges is exceptional. Spring brings occasional wildflowers to the rocky plateaus between the Atlas and the desert.
Summer (June to August) is hot and can be extreme: midday temperatures routinely exceed 40°C, making sightseeing difficult and the desert uncomfortable outside the early morning hours. If you visit in summer, plan all outdoor activity for before 10am or after 4pm.
Winter is cold at night — Ouarzazate's altitude means temperatures drop sharply after dark, sometimes below freezing in December and January — but the days are often bright and clear. The Atlas passes to the north can close in heavy snow, which may affect overland travel from Marrakech. However, winter is also low season and the town is very quiet.
The Draa Valley date harvest happens in autumn, roughly October to November, when the palmeries south of Agdz and Zagora are at their most productive and the valley roads are lively with local traffic. There are no major festivals to time a trip around, but if you are combining Ouarzazate with Marrakech, be aware that the two cities share the same peak tourist windows in spring and autumn.