| For visiting Yellowstone in the "off season", the main consideration is road closures. Most Yellowstone park roads are closed from November 4 through December 14. During that time, almost all park facilities are closed. The road from the north entrance to the Mammoth Hot Springs Visitor Center remains open all year. From mid-December through February park travel is only possible via snowmobile and snowcoach (again, except for the north entrance / Mammoth area). Then all roads close again in March. Roads open to normal cars again starting at the end of April and into May. Snow accumulation and animal activity can keep some roads and trails closed into June. More info from the Park Service:
https://www.nps.gov/yell/planyourvisit/parkroads.htm --- --- --- When to visit: Best wildlife watching: spring. Fewer people and lots of wildlife out with their young. Best geyser watching: the hottest, cloud-free day of summer. Hot water makes lots of steam, so eruptions are more difficult to see in cold weather. Similarly, geyser eruption plumes have low color contrast with respect to clouds. Also, see: https://geysertimes.org/ On balance, best time to visit: September and October. Fewer people. Roads and trails usually open. Days can possibly be warm. Plenty of wildlife. |
In general, good advice for a lot of Western US parks and other areas. You're out of the peak vacation months so fewer people. But weather is still mostly good, bugs are mostly gone (though that's not as big a problem in the West as elsewhere), it's getting cooler but snow isn't really a problem yet, etc. September to early October is when I plan a lot of vacations when I can.