Arequipa is essentially a year-round destination. The city sits at 2,335 metres on the dry western slope of the Andes and gets over 300 days of sun a year. Daytime temperatures rarely move much from 20-23 °C, and the rainy season — December to March — produces only short afternoon showers that mostly clear by evening.
May to August is the dry season and the most reliable time to visit. The downside is colder nights (close to freezing in July) and dry, dusty air. December to February is warmer and greener but means a real chance of rain affecting any side trip into the mountains. April and October are sometimes the best months: warm days, low rain risk, fewer tour buses.
The big festival is the Anniversary of Arequipa, 15 August, with parades, fireworks, and the locally famous corso on the day itself — book accommodation well in advance.