Namur has a mild continental climate — cool, wet, and given to grey winters. May to September is the best stretch for visiting: long evenings on the riverside terraces, walkable old town, and citadel grounds at their greenest.
The big city festival is the Fêtes de Wallonie in mid-September, when the streets fill with concerts, parades and the famous stilt fights (the Echasseurs Namurois) — teams of men in seventeenth-century dress duelling on wooden stilts on Place Saint-Aubain. It is unhinged and entirely local; book accommodation in advance.
For a quieter visit, come in April–June when the citadel gardens are flowering, or October, when the surrounding forests turn. The Christmas market along the Sambre quay is small but charming.