The dry season — May through October — is the best time to visit Cusco and the surrounding region. Days are clear and sunny, nights are cold (temperatures drop to 0–5°C even in June), and the Inca Trail is open. July is peak season; the city is crowded and Inca Trail permits sell out months in advance.
The wet season runs from November through April. Rain falls most afternoons, the Inca Trail closes in February for maintenance, and some mountain roads become impassable. But the crowds are thinner, prices are lower, and the landscape turns lush and green. January and February are the rainiest months; March and April are wet but transitional.
The Inti Raymi festival of the sun, held on 24 June, is the year's biggest celebration and draws enormous crowds to Sacsayhuamán. Book accommodation months ahead if you want to be there.