Peru has fairly definitive times for travelling with specific seasons, but travelling during the border months can be the most rewarding.
From March to November is the rainy season for the country. During these months you will find warmer night time temperatures and low lying cloud. April to October is the dry season but in both April and October you can still expect a few showers.
For us, the middle of March to the start of May are some of the best months as all the wild flowers are out, especially the Orchids and there are fewer travellers around as well. The same can be said of October, a time when there are fewer travellers and the first rains can bring a different colour to the country.