best time to go yellowstone

We were there middle of may a year ago. Weather was high 50's low 60's during the day.At night it dropped into the high 30's. Crowds were light. Not everything was open, but there was more than enough to see. We had buffalo walk so close to out van we could have reached out and touched them. We also had a mother grizzly and 3 cubs walk right toward us and then passed aside of us. We thought it was a great time to be there. We were there 4 days and could have used 2 more. We stayed in Madison campground. No hookups but generators allowed. It is centrally located. We dined at the Inn at Old Faithful. Beautiful lobby. The food was good and the price was decent.Mammoth Hot Springs at the north end has a lot to see and also has a nice campground.The best part is that we never hit a traffic jam and could make good time getting from one place to the next.So I say go in May!