The M'zab Valley is in the northern Sahara and experiences extreme temperatures. Summer (June to September) is genuinely difficult: daytime highs above 40°C are common, and the lack of shade in the old ksour makes walking outside peak hours punishing. October to April is the best window, with pleasant daytime temperatures and cool evenings. March and April bring some risk of sandstorms but are otherwise excellent.
The weekly market days (Thursday in Ghardaia, Friday in Beni Isguen) are worth timing your visit around if possible. Ramadan makes the Mozabite communities somewhat more closed than usual to outside visitors.