San Pedro de Atacama is open year-round and visited year-round. The climate is extreme but predictable: intense sun, cold nights, low humidity, and very little rain for most of the year. The main seasonal variable is the Bolivian Winter, the period of altiplano thunderstorms that brings rains to the high desert between November and March. During this period, some roads to high-altitude sites may be temporarily closed, and the landscape around the salt flats and lagoons can look dramatically different — with water where there was none, flamingos where they weren't, and dramatic cloudscapes over the volcanoes.
The dry season from April to October is the most reliable for visiting all sites without weather-related closures. July and August are the busiest months and can be cold at altitude (below -10°C at El Tatio at dawn). The spring shoulder seasons (September–October, April–May) offer good conditions, fewer crowds, and the most pleasant weather.
For stargazing — one of San Pedro's great attractions, given the altitude, dry air, and minimal light pollution — any time of year is excellent, though the Milky Way is most spectacular in the southern winter.