True. I went in july 2010, dry season, but at night very, VERY cold (specially for me, a Lima guy).

By the way, if you want to drive to Chivay, don't go during rainy season. It can be complicated (landslides) and dangerous. Between late may and early september is the best time, dry season.

Another tip, the Casa Andina hotel in Chivay has a small astronomic observatory. If you are not a guest the entrance is cheap, and they do interesting sessions explaining (and watching) the southern hemisphere constellations. But you have to book early, email the hotel in advance.