Peru can be visited all year round, but it all depends what do you want to do or visit.

Summer is from December to March but, it is the rainy season on the highlands and the jungle. During February the Inca Trail is closed for tourists.In summer the beaches south of Lima are great. You can also go to the North of Peru and visit Mancora all year round, as it is close to the Equator and will be sunny.

It never rains in Lima, but it gets cold due to the humidity between June and August. Its normally overcast with fog. If you go out of Lima towards the central mountains, you will find the sun about an hour away.

Most popular time to visit Cusco is between June and August, but the recommend time to visit is in April, May or September to October, just after and before the rainy season and the tourist crowds. If you do not mind the rain and are not doing the Inca trail, the highlands are beautiful green and flowery during the rainy season.

The sun is always out in the highlands, but it will be chilly in the afternoons and quite cold during winter at night. Dress  with clothing in layers ( t shirt, long sleeve shirt, sweater, jacket) so you can "peel" them off as it gets warmer and put them back on when it gets colder. Bring a small backpack to carry your clothing and tourist apparel.

 For 10 days extended forecast in English, in the main cities of Peru go to : http://www.senamhi.gob.pe/