Rissani is on the edge of the Sahara and the climate is unforgiving for half the year. October to April is the only sensible window — daytime temperatures are warm but not punishing and the nights can be cold enough to want a fleece. The desert excursions south to Merzouga are at their best in November and February when the air is clearest.
May to September is genuinely difficult: daytime temperatures of 45°C and above are routine in midsummer and the wind off the Sahara turns hot and gritty. Most of the trans-Saharan tour groups pull out completely by mid-June.
The date harvest runs in October and is the most interesting time of year for the souk: trucks rattle in from the surrounding oases laden with crates of medjool dates and the auctions in the date market run all morning. The Moussem of Moulay Ali Cherif is held in late summer or early autumn (the date shifts with the Islamic calendar) and draws pilgrims from across Morocco; lodging in Rissani is impossible without an advance booking during the festival.