HI. . Can I visit wu dang Shan in Dec ? will it be very cold and having snow ? Any tour arrangement

Yes, you can visit in Dec. but it will be cold, especially on the mountaintop where the lowest temp may drop to -10C or around. It is necessary to prepare warm clothing.

Only for the mountain, it is no need to join a tour. Anyway, if you wanna make everything smooth, stay in a hotel in Shiyan and ask them to arrange a comfortable one for you.