The timing of a Mudanjiang visit is almost entirely determined by Snow Town and Jingpo Lake — and these two attractions favour opposite seasons.
Winter (December–February) is the peak season for Snow Town, when the village's legendary snowfall is at its deepest. Temperatures in Mudanjiang itself drop to -20°C or lower; that is cold, but the city has good indoor infrastructure and the journey to Snow Town is perfectly manageable in the right clothing.
Summer (June–August) is when Jingpo Lake is most accessible and attractive. The water is navigable, the forest is green, and the boat trips run frequently. The waterfall is at its most powerful after the spring snowmelt. Mudanjiang in summer is warm, occasionally hot, with temperatures reaching 30°C.
Autumn (September–October) offers an underrated window: Jingpo Lake's deciduous forest turns yellow and orange, the tourist crowds thin considerably, and the weather is clear and cool. This is arguably the best time to visit if you have flexibility.
Spring (April–May) is muddy and damp, and Snow Town's snowdrifts are melting. There is little reason to visit then unless you are passing through.