best time to travel Glacier, Yellowstone, Badlands

Spring after snow is mostly gone and before schools get out.
Autumn before snow flies and after school starts.
Yellowstone and Tetons in the fall. Elk buggling is something to behold. Fall colors in the trees.
Spring with all the baby animals and the wild flowers.
Badlands can get hot during the summer. Late spring and fall.
Confused? I didn't help any. Two trips would be my choice if you have never been there. OH! Wait! Do both and extend your trip. Early May and come home in October.