Yellowstone National Park in December

Summer means crowds, but it also means all roads are open and wildlife is active. July and August see the heaviest visitation — expect traffic jams around Old Faithful and full parking lots at popular trailheads by 9 AM. September is the sweet spot. Crowds thin out after Labor Day, elk are bugling, and aspen trees turn golden in the high country. You'll still have warm days but crisp nights that require a jacket. Early summer brings wildflowers and baby animals, but also unpredictable weather. I've seen snowstorms in June that close mountain passes. Late summer offers the most stable weather and the best hiking conditions. Winter transforms Yellowstone into a different planet. Only the road from the North Entrance to Cooke City stays open to regular vehicles. Everything else requires snowcoaches or snowmobiles. But winter wildlife viewing can be spectacular — bison create their own trails through deep snow, and the geysers look even more dramatic surrounded by ice and snow.