See January, February, March, and April. May is a great month to visit many parks, especially river and forest parks that are full of life in spring. I recommend Congaree, Cuyahoga Valley, New River Gorge, Olympic and Yosemite.
Congaree is a great place to kayak, so high water may allow you to roam more. But May is also when the fireflies all blink together. Recreation.gov holds a lottery in early April for evening passes during the week long synchronous firefly event mid May.
Cuyahoga Valley is popular in May as the Brandywine Falls are awash and the trails are surrounded by spring greenery. I like biking there, and the tourist trains run a full schedule in May also taking bikes.
New River Gorge may be best in May as the wildflowers are out on the trails, the temperature is just right, and the river runs fast for rafting.
Olympic for me is all about the rainforest. Go early, wear good hiking boots, a thermal layer and rain gear, but try to visit Hoh Rainforest and hike the Hall of Mosses when it’s wet and green. Visiting the rainforest is a unique national park experience, unlike mountain hikes or beaches. The park is beautiful during the summer, but it gets very crowded at Hoh beginning Memorial Day weekend.
Yosemite’s waterfalls all flow strong in May, and Bridalveil, Sentinel and Yosemite Falls are spectacular at peak snowmelt. Wildflowers bloom on the valley trails, the temperature is pleasant during the day, and it’s not as crowded as summer. Glacier Point viewpoint usually opens to vehicles in May, earlier now that there’s less snow.
You may notice that my recommendations on when to visit are often a little earlier than common advice, and that’s usually to avoid crowds, to take advantage of planning and because the climate is changing. Planning ahead is particularly important for saving time and money, as a simple campground reservation made six months in advance can put you in the best possible location at the best time for $30. Summer used to be the best time to visit most national parks, but many parks are now too crowded and too hot in July and August. But stay tuned, because I’ve got more national parks recommendations coming on the first Friday of each month.