Only the sky knows when it will snow :)
Ski resorts further away from Seoul, such as Vivaldi, Phoenix, Alpensia, Yongpyong usually opens a bit earlier in last week of November. Ski resorts close to Seoul, such as Yangji Pine, Gonjiam, Elysian usually opens later, around early December.
This schedule may change last minute due to the weather situation. If the temperature falls below 0, the resort will make artificial snow in order to open the resort.
I suggest you to book the tour first so we can inform you if there are any changes in the ski slope opening schedule. If the package gets canceled due to ski slope not opening, full refund will be issued.
2018 January is Pyeongchang Olympic Games season so some resorts will not be open to the public due to the Games. Booking early in advance is recommendedd especially for this winter!