The Moussem of Moulay Idriss II is Fez's most important religious festival — a week of ceremonies, music, processions, and communal prayer at the shrine of the city's patron saint every September. The fantasia (horsemanship display) and the Sufi music sessions that accompany the moussem attract visitors from across Morocco. The final procession through the medina, with decorated floats and costumed participants, is one of the most spectacular public events in the country.
During the moussem, accommodation in Fez fills completely and prices rise significantly. If you happen to be in Morocco in September, the festival is worth timing your visit around — the medina at moussem time, dense with pilgrims and ceremony, is the city at its most intense. Dates vary each year by the Islamic calendar; check locally.