We were there mid May 2016.Great time. Weather was high 50's low 60's during the day. Dropped into low 40's high 30's at night. Crowds were light.No traffic jams.Not everything is open but enough to make it worth while.We stayed in Madison Campground. I would advise reservations.Madison allows generators during certain hours. It is centrally located. Yellowstone is big so staying in the park will save time. Make sure you visit the Inn at Old Faithful. Beautiful lobby and the food was excellent and affordable.We saw a mother grizzly and 3 cubs on the road from Norris to Canyon.The south entrance wasn't open when we came in but was open when we left. There was snow on the ground in the south end of the park and some ice on the lake too. But we felt May was good time to visit.