We went in mid May of 2016.It was a great time.Weather was high 50's,low 60's during the day.At night it dropped down to the high 30's. Crowds were light. We stayed in Madison Campground in the park. It doesn't have hookups but generators were allowed. It saves a lot of time staying in the park.Not everything is open, but there's enough to make to worth while.I would suggest eating at the Inn at Old Faithful. The lobby is something to see.The food is good and prices are reasonable.I would suggest buying books on Yellowstone. That way you would know what to expect.