The valley is worth visiting year-round, but the season matters. Spring (March–May) is lush and green, with wildflowers along the river terraces and snow still visible on the upper peaks. It is the prettiest time, but snowmelt can make the track beyond Setti Fatma impassable and the river runs fast and high — take care near the water.
Summer (June–September) is the peak season for Moroccans escaping Marrakesh's fierce heat. The valley stays noticeably cooler than the city — 10°C or more in high summer — and the roadside restaurants and guesthouses fill up at weekends. Come midweek or early morning to beat the crowds.
Autumn (October–November) is probably the best compromise: clear skies, comfortable temperatures, fewer people, and the river low enough to make trekking safe and easy.
Winter brings snow to the higher elevations and cold nights, but the valley road itself is usually passable and the midday light is good. The landscape is stark rather than lush.
Avoid the valley on public holidays and summer weekends, when the single road in and out can turn into a slow-moving queue of cars and minibuses.