El Oued is one of the hottest cities in Algeria, and the difference between visiting in winter and visiting in summer is brutal. From May to September, daytime temperatures regularly exceed 40°C and often top 45°C; July and August can hit 48°C or more, and the desert wind can push the temperature higher. Most desert excursions don't run in summer, and ordinary sightseeing becomes a question of finding shade between dawn and dusk.
November to March is the time to come. Daytime temperatures sit between 15°C and 25°C — pleasant in the sun, cool in the shade — and nights can drop close to freezing in December and January. Pack layers. The light is at its best in winter too, with long shadows over the ghout craters and dunes.
October and April are shoulder seasons: warm but tolerable, and lighter on tourists than the deep winter months.
The Thursday market is the biggest day of the week; if you can plan your visit to overlap, the souk is at its most lively. The main religious festival of the year is the ziyara of Sidi Salem, generally held in spring (date varies with the Islamic calendar) — worth attending if you can time it but expect the town to be packed.