The Cusco region has only two distinct seasons: the wet season and the dry season. The rainy season begins in October when in the southern hemisphere spring begins and ends in March. At this time the rain falls almost every day for three or four hours, mainly between December and March, but there are also several days of sunshine The dry season starts around March and lasts until September, during this time almost no rain and the sky it is completely cleared every day. However, between May and July it is very cold. In Cusco these months are called "frosty weather".
But if you want to find a bright sun very early time and almost no rain, the best time is between June and October.
It should be noted that from June to August is the high season, where obviously find tickets, hotel reservations, among other things, with some high prices and will need to book accommodation or trips months in advance, but with a good tourism agency this will be easier.
A good option may be to visit Cusco between April and May, just after the rainy season but before the start of the high season. And the weather also will be nice and warm.
The rainiest month is definitely February when usually have to close some routes to Machu Picchu due to heavy rains along the way and for manteniemiento this Inca treasure that is Machu Picchu
Many people wonder if it is safe to walk during this season, and most of the time is fine. The rain is sporadic, usually is not constant and is not as bad as it seems, can always enjoy Cusco and its wonders any time of year.