Altena is a year-round destination in the sense that the castle and museums are open throughout the year, but the town is at its most pleasant from May through October when the Sauerland forest is fully in leaf and the hiking trails are at their best. The beech and oak woods above the Lenne valley turn a good colour in October. Summer weekends see more visitors to the castle, but Altena never becomes crowded by any reasonable standard.
Winter visits are quieter and the castle in low light has a suitably medieval atmosphere, though some trail sections can be muddy or icy. The Christmas market in Altena is small but genuine. Avoid the castle restaurant at weekends if you dislike queues.
There is no bad time to visit if the main reason is the youth hostel museum and the Drahtmuseum — both are open year-round and are indoor attractions that weather cannot affect.