2020-2021 Winter Opening Dates. Conditions permitting, roads will open to oversnow travel by snowmobile and snowcoach at 8 am on the following dates: December 15: West Entrance to Old Faithful, Mammoth to Old Faithful, Canyon to Norris, Canyon to Lake, Old Faithful to West Thumb, South Entrance to Lake, Lake to Lake Butte Overlook. December 22: East Entrance to Lake Butte Overlook (Sylvan Pass)