Peru has no single best season, because it has three climates running at the same time. What is high season in the mountains is grey winter on the coast.
The highlands set the calendar
If your trip includes Cusco, Machu Picchu, Titicaca or any trekking, the reference is the dry season: May to September. Sunny days, clean skies, cold nights — below freezing on the altiplano — and very little rain. It is also when there are most people and the highest prices, especially in July and August.
From December to March it rains daily in the highlands, with heavy afternoon showers. Machu Picchu stays open, but the Inca Trail closes in February and high-altitude trekking gets complicated. In exchange the landscape is green and there are far fewer people. April and October are excellent transition months: still-settled weather and lower prices.
The coast runs the other way
Lima and the whole littoral live under the garúa from May to November: low fog, high humidity and little sun, though almost no rain. Coastal summer runs December to March, hot and clear, and that is when the northern beaches at Máncora or Punta Sal make sense — although those are warm all year.
The rainforest
It always rains, but from December to March far more. The dry season makes walking easier and concentrates wildlife on the riverbanks; the wet season raises the rivers and opens up routes between flooded trees.
Altitude matters more than the month
Cusco is at 3,400 metres and Puno at 3,800. Flying straight from Lima to Cusco and setting off walking the same day is the recipe for soroche. Plan two acclimatisation nights before any effort, or start in Arequipa (2,335 m) and the Sacred Valley (2,800 m) and climb afterwards. Drink water, eat lightly and avoid alcohol for the first forty-eight hours.
Festivals that dictate dates
Inti Raymi on 24 June fills Cusco completely; book far ahead or avoid those dates. Puno's Virgen de la Candelaria in February is one of the largest dance festivals on the continent. Fiestas Patrias, on 28 and 29 July, moves the entire country.
A simple rule
If the highlands are your priority, travel between May and September and accept the crowds. If the coast matters more, go between December and March. For a balance of decent weather, moderate prices and fewer people, aim for April, October or early November.