The best time to visit the lower Omo valley is October to February — the long dry season — when roads are passable and the market gatherings of the different peoples are most regular. The short rains in March and April can make roads impassable, and the long rainy season from June to September renders much of the valley inaccessible.
For rafting the Omo River, October to January is the prime window: the rains have eased, the river is running well, and the gorge sections are navigable. Avoid March to September for river expeditions.
The weekly markets are the best opportunity to see multiple groups in one place: Turmi on Monday, Dimeka on Tuesday and Saturday, and Key Afer on Thursday are among the most active. Timing your visit around these markets is worth doing.