How long do you have to get there--and then to stay. I like the drive to Salt Lake City and then up, spending some time in the Tetons also.

First thing to know about Yellowstone in May: it's pretty early in the season. Some major facilities won't open til mid-month.
www.nps.gov/yell/planyourvisit/open_closedates.htm#CP_JUMP_127493

Trails will be snow-covered at best, and many will be closed.
If you only want to drive around the road, this may be a good time to go.

> Any advice?
http://www.nps.gov/yell/planyourvisit/index.htm
http://www.yellowstone-natl-park.com/
http://www.ohranger.com/yellowstone/toc/overview-map

If you would like a scenic route as opposed to fastest, then I would go from Denver to RMNP. Travel across the park east to west on Trail Ridge Road. Take 34 to 40 through Steamboat Springs, crossing Colorado to Vernal,UT. Head north through Flaming Gorge RA to Grand Teton NP, then on to Yellowstone. The fastest route, 25 north to 80 west is pretty boring.

Unfortunately Trail Ridge road probably not be open until right at the end of May

It looks like federal budget cuts will delay openings at Yellowstone this year:
http://www.fodors.com/community/united-states/yellowstone-opening-delayed-grand-teton-visitors-centers-to-stay-closed.cfm

Lee Ann

The target for opening Trail Ridge Road is always the Memorial Day weekend. Most years they make it and a few times they don't. All depends on snow conditions and especially late season snow. May can still produce iffy weather in the mountains. On average May is wettest month in Denver which can mean snow in the mountains. Not as severe as Jan/Feb but still troublesome.