Hi Willi:

How do you know? Well I don't know much.
That's the 4th time I've gone up there. The later the
better is the local wisdom. If I could choose I'd look for
good weather in early November.
But they close the Park to fishing Nov 1.

And you have to reserve a campground months in advance.
That's the worst part. It always seems to be bad weather
when you have no date flexibility.

One interesting thing: the Park service keeps (only) Yellowstone
Lake open past Nov 1st. As long as the roads are open, you
can fish. And if you do catch a laker, you cannot release it.
You have to konk'em whether you like it or not,
and there is no limit. And the lakers do come into shallow
water in late October. Sort a redneck fish konkers holiday.