The best times to visit Fez are spring (March to May) and autumn (September to November). Temperatures are comfortable — 18–25°C — the light is excellent, and the city feels busy without being crushed. The medina is always crowded in summer, but the heat (often above 35°C in July and August) makes it genuinely exhausting.
Winter (December to February) is cool and sometimes rainy, but Fez winters are mild compared to northern Europe and the crowds are thinnest. You will have the medina largely to yourself in January, which is its own reward. The Merenid Tombs and the hilltop viewpoints can be cold and windy.
Ramadan transforms the city in ways that are worth experiencing if you understand what you are getting into. The medina slows dramatically during daylight hours — many restaurants are closed, shopkeepers are fasting and sometimes short-tempered — but after iftar (the sunset breaking of the fast) the city comes spectacularly alive. If you visit during Ramadan, respect the fast in public spaces and expect that business hours will be irregular.
The Festival of World Sacred Music (Festival de Fès des Musiques Sacrées du Monde) takes place in June and is one of the most worthwhile cultural events in Morocco, drawing Sufi musicians, gospel choirs, and sacred music traditions from across the world for concerts in the medina and the Bab Makina square. Tickets are required; the festival fills accommodation quickly.