Yellowstone National Park’s Sound and Video Libraries

Depends on your tolerance for cold weather. I was planning a camping trip in October a couple of years ago and ended up not doing it because while I could have managed, I wasn't really prepared to deal with sub-freezing high temperatures and it ultimately didn't sound like much fun anyway. On the flip side, I've heard that Yellowstone is one of those places that is so popular that it's overrun with tourists to the point of no longer being fun during the peak season. I think the best time to go is therefore either very early in the spring or very late in the fall when it's cool, but not miserably cold.

Fall is best. Crowds are down, the critters are actively prepping for winter, and the rangers haven’t turned off the geysers yet ;). Only real issue is winter is coming and you never know when. Dress warm.

You may be interested in this lovely visualization:

http://jordan-vincent.com/night-under-the-stars

It shows that Nov-Dec in Yellowstone has the lowest number of visitors in any type of accomodations. Also, average temperatures under 50F/10C for most of November and all of December - and those months will not have a winter's worth of snow base slowly melting off as it would in March, they'll just be cold.

Take that as you will: either the busy park will be blissfully quiet and it's a great time to go, or a lot of people think it's a bad time to visit.

Looks like Joshua Tree, CA and Big Bend, TX are more popular parks in those months.

Yellowstone closes to wheeled traffic in about early November and doesn't open for winter (snowcoach)until about mid-December.

Winter is great if you have the clothing/equipment/interest but Nov/Dec isn't the best timing and, even though it's open over Christmas or thereabouts, I've been told that animal viewing tends to be better later in the winter season.

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

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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.