The best time to visit Ancash for trekking and mountain activities is the dry season, May to September. July and August are the peak months — skies are generally clear, the days are warm at altitude, and nights cold. The Santa Cruz Trek and Laguna 69 day hike are both best done in this window.
October to April is the wet season, with afternoon rain and occasional snow at higher altitudes. The valleys turn green and the wildflowers at lower elevations are excellent, but trails become muddy and some routes difficult. Chavín de Huántar is visitable year-round and the rains make little difference to the underground galleries. Altitude is a factor at any time of year: spend at least two nights in Huaraz before attempting any serious hiking.