Peru has 2 distinct seasons. The dry season starts in mid April and ends mid November. Rainy season takes full hold December – March. For obvious reasons, the dry season is the ideal time of year for trekking the Inca Trail.
December to April: The rainy season starts in December, and continues to March. January and February are the rainiest months. In fact, the Inca Trail is closed for February. December and March have rain nearly every day, but usually not a ton of rain all at once. April can go either way. It can have rain or beautiful weather.
May to June: These months are usually warm and dry. As the rainy season isn’t too distant, the landscape is still lush and green. July to November: These months are dry and cool. The landscape starts to brown. You might start getting a bit of rain in November.
Inca Trail permits sell out the quickest for the months of April through August. Permits will sell out sometimes 6-8 months in advance.
NOTE: Keep in mind Cusco, the Sacred Valley, and Machu Picchu are high in elevation, and the weather can change at any time. While these descriptions of the weather are the general trend, you should always be prepared for hot sunshine, cold rain, and everything in between.