ps I forgot to add... our summer isn't the best time to go... their spring is around Sept-Nov and probably best weather wise. I once spent some time in Argentina in high summer and it got unbearably hot. Depending where you are going in Peru, there is a rainy season... a quick google turned up this..
Travelers can visit Peru any time of the year. Dry season runs from May to November and this is typically the time that is most recommended. However, this is also the cooler time of year. Nighttime temperatures can drop to below freezing at the height of the dry season. June, July and August are the most popular months to visit so you will tend to encounter much larger crowds during these months.
In the wet season (December to April), you can expect showers three to four afternoons a week. For travelers that don't mind a little drizzle and muddy trails, this time of year offers smaller crowds and greener hillsides, with wildflowers and orchids often in bloom.
The shoulder seasons, April to June or September to November can often provide the best of both worlds. They typically have fewer crowds and warmer temperatures than the height of the dry season, but still tend to have relatively little rain. For more information, check out our Peru Weather page.