Jyväskylä is at its most appealing in summer, from June to August. The lakes are warm enough to swim in, the long evenings are ideal for outdoor cafés and cycling around Jyväsjärvi, and all the museums and outdoor attractions are fully open. July is the peak month. Summer brings the Jyväskylä Rally (part of the World Rally Championship), typically held in late July or early August — a spectacular event if you don't mind crowds, and one that books out accommodation city-wide.
Winter (December through February) is cold, dark, and snowy — the lakes freeze, and there is some cross-country skiing and skating. Spring (April-May) is when the ice breaks and the lakes come back to life; it can be wet but is beautiful when the birch trees come into leaf. Autumn, particularly September, can be excellent — the forest colours are vivid and the crowds are gone.