Jilin City is most famous in winter, and if you have any tolerance for cold, December through February is the best time to visit. The rime ice on the Songhua River forms when conditions are right — typically several times through the winter — and the city's winter character is unlike anywhere else in China. Temperatures can fall to minus twenty-five or below; dress accordingly.
Summer is pleasant for the Songhua Lake boat trips and Beishan Park, and the river is a different but agreeable presence in the warm months. Autumn (September to October) brings good weather and the beginning of the cold season without the extreme temperatures.
Spring is the least appealing season — the transition out of winter is damp and grey, and the rime ice has gone but summer has not yet arrived.
For Beidahu skiing, December through March is the season, with January and February the coldest and the snow conditions most reliable.