Please let me know something about Changbai Mountain:

1. There are buses between northern and western entrances. The buses depart from northern entrance at 8:15 and 14:30. The bus departs at 10:40 from western entrance. A single trip takes about 3.5 hours.
2. You may go to the northern entrance.
3. There are hotels at northern and western entrances.
4. You may spare two or three days there.
5. Yes, you can pay a visit to the mountain and the lake in December. You will see a fairy land of ice and snow. But it will be quite cold. You should pack down jackets, thick sweaters, thermal underwear, warm pants, boots, scarves, and gloves. Keep warm and have fun! :)