Yichun is a summer destination for most purposes. The severe winter is part of the city's character but places real practical constraints on what is enjoyable.
Summer (July–August) is the best time. These are the two genuinely warm months in Yichun — temperatures reach 25–28°C during the day, the forest is at its most lush, and the clean air after the long winter is striking. The city is at its most functional, the markets are full of forest products, and the hiking trails in the surrounding parks are all accessible. This is the correct time to visit.
Late spring (June) is cool (sometimes uncomfortably so — temperatures can still drop below 10°C at night) but the forest is brilliantly green after the snowmelt and the spring wildflowers in the forest understorey are remarkable. A good secondary window.
Autumn (September–October) brings the first frosts in September and the leaves turn gold and orange by October. The forest in autumn colour is beautiful, the air is clear and cold, and the tourist pressure drops sharply. Bring warm clothing.
Winter requires serious commitment. Temperatures drop to -40°C and the city quiets considerably. The forest in heavy snow is beautiful in a genuinely Siberian way, but most visitor infrastructure is reduced. Only if you specifically want to experience subarctic winter.