The Sacred Valley has the same broad dry/wet pattern as the southern Andes generally. The dry season (May to October) is the standard window, with clear skies, cold nights, and warm days — peak tourist season runs June through August. The wet season (November to April) brings afternoon downpours and slippery trails, with the Inca Trail closed during February for maintenance.
The valley sits 400 to 600 metres lower than Cuzco, which makes it noticeably warmer and gentler at any time of year — a real advantage for travellers acclimatising. Many people now spend their first two or three nights in the valley before heading up to Cusco, rather than the other way around.
Key festival dates worth timing around include Inti Raymi (24 June, the Inca festival of the sun, biggest in Cuzco but with parallel celebrations in the valley villages) and the Sunday market at Pisac, which is at its busiest and most colourful around festivals.