Spring and autumn are known for the mild weather and fewer crowds, that's why April to May and September to November is the best time to visit Yellowstone National Park. In spring, the weather is unstable, and you need to pack layers. Most roads open by the third week in April, and it's about time to visit unique Isa Lake. Autumn in Yellowstone offers you beautiful orange-yellow foliage, a swim in boiling river, and amusing moose mating season, but most facilities close by early November. Peak season between July and August brings lots of summer activities and perfect weather to visit Grand Prismatic Spring and Morning Glory Pool. However, summertime can be unbelievably busy, so you'd better book your room in advance. Winter in Yellowstone is a harsh and snowy period when some facilities and roads are closed. Anyway, you still have an opportunity to observe migrating bison and wolves, enjoy winter sports, and witness steam rising from the thick blanket of ice and snow and droplets freezing in the air.