What will the weather be like in Chengdu during the month of December?

Hi, welcome your asking.
In the early Dec the average temperature of Chengdu could be about 10 C, while from the middle to the late Dec it can only be about 6 C. So in general, it is cold then. Maybe it would snow then. You need take along some warm clothings.
For Jiuzhaigou at that time, some areas would be closed for the cold weather or the snow. To be honest, it is not a good time to visit Jiuzhaigou. Anyway, it is still open then.