There are two main seasons in Peru: the rainy season and the the dry season. The rainy season runs between October and April, with the dry season usually running between May and September. Temperatures during the dry season run from around 12 C (50 F) at night to the mid to 30 C (upper 80s F) during the day. Given its exposed location, it can get quite hot in Machu Picchu, and at its elevation, the sun is strong -- do not forget your sunscreen!
The dry season is the best time to travel to Machu Picchu -- and not surprisingly, coincides with the high season. To avoid the crowds, consider traveling in late April, May, June, or early October. Crowds are the heaviest during July and August.