It could be said that in Cusco we have 02 marked seasons:

Dry season: From April to September, with quite hot days, blue sky, cold early in the morning, windy in the afternoon. Good weather for adventure tours such as the Inca Trail and other trails.

Rainy season: from September to March, rainy days, requires a good raincoat.

Stations of Cusco in detail.

December 21 to March 21 Summer.

In Cusco it is very rainy, of warm climate in reference with the winter, but rainy, also there are days of sun, it can be shown of morning a very sunny day and in the afternoon to surprise with an intense rain, or it can also be shown the day with a rainy dawn and then be transformed into a sunny day, reason why in this period it must bring a good raincoat, clothes of coat and also clothes of sun.

March 21 to June 21 Autumn.

The rains continue with intensity, until the last days of March, in April there is a change, the dry season begins, the nights and mornings are cold, the days are sunny, from the first day of April it is recommended and very good to schedule walking tours, bring the appropriate clothing.

June 23rd to September 21st Winter.

The cold begins in the afternoons, nights and early morning, the cold arrives below 7 degrees below zero during the night, the days are very sunny with blue sky, short, the night arrives at 17:50hrs, are very clear nights with spectacular views of the stars.
Recommended, great for hiking tours.
Bring warm clothes and sun clothes, hat, sunscreen, moisturizer.

September 21 to December 21 Spring.

Cool weather, beginning of rains that are increasing until the end of March.