May is a turning point: snowmelt powers Yosemite's waterfalls, Great Sand Dunes gets a seasonal beach, baby bison appear on the northern plains, and the first summer programs start.
Yosemite's waterfalls usually peak in May and June with snowmelt. Yosemite Falls roars, and the Mist Trail earns its name. No reservation is needed to enter in 2026.
Hikes to do: Mist Trail to Vernal Fall, Lower Yosemite Fall, Mirror Lake, Mariposa Grove Loop
Don't miss: Yosemite Falls from the valley floor, Vernal and Nevada Falls via the Mist Trail, Mariposa Grove of giant sequoias
Tip: Half Dome cables typically go up around Memorial Day weekend. Permits are by lottery (see Permits & reservations).
Late May into early June, Medano Creek flows across the base of the dunes, creating a seasonal beach with surging waves. It's the park's signature event and it only happens now.
Hikes to do: High Dune, Dunes Overlook, Mosca Pass Trail, Montville Nature Trail
Don't miss: Medano Creek at the base of the dunes, High Dune, Zapata Falls just outside the park
Tip: Sand surface temperatures can exceed 140°F on sunny afternoons. Climb the dunes in the morning.
Ranger-led cliff dwelling tours open for the season (May 4 in 2026), and Wetherill Mesa opens later in May. Spring temperatures are ideal on the mesa tops.
Hikes to do: Petroglyph Point, Soda Canyon Overlook, Point Lookout, Knife Edge Trail
Don't miss: Cliff Palace and Balcony House on ranger tours, Spruce Tree House overlook, Wetherill Mesa (opens late May)
Tip: Tour tickets release 14 days ahead at 8 a.m. Mountain time and sell out. See Permits & reservations.
Synchronous fireflies flash in unison in Congaree's floodplain forest from about mid-May to mid-June. It's one of only a few places in North America where they happen.
Hikes to do: Boardwalk Loop, Weston Lake Loop, River Trail
Don't miss: Boardwalk at dusk for fireflies, Weston Lake, Cedar Creek canoe trail
Tip: Mosquitoes are ramping up. Bring repellent and check the park's alerts for firefly event info.
Bison calves, prairie dog pups and green badlands. Late spring is when the North Dakota badlands look most alive, and summer crowds haven't arrived.
Hikes to do: Wind Canyon, Buck Hill, Painted Canyon Nature Trail, Caprock Coulee Loop
Don't miss: Painted Canyon overlook, Wind Canyon at sunset over the Little Missouri, Prairie dog towns along the park roads
Tip: Give bison at least 25 yards, more with calves.
Bat flight season begins as hundreds of thousands of Brazilian free-tailed bats spiral out of the cave at dusk. It runs roughly late May through October.
Hikes to do: Natural Entrance, Big Room Loop, Chihuahuan Desert Nature Trail
Don't miss: Evening bat flight at the Natural Entrance amphitheater, The Big Room, Walking down through the Natural Entrance
Tip: A timed-entry reservation is required to enter the cavern. See Permits & reservations.
Mountain laurel and azaleas bloom along Skyline Drive, the forest is fully green, and waterfalls still run strong from spring rain.
Hikes to do: Dark Hollow Falls, Stony Man, Whiteoak Canyon, Old Rag Mountain
Don't miss: Big Meadows, Skyline Drive overlooks, Dark Hollow Falls
Tip: Old Rag requires a day-use ticket March–November. Tickets release 30 and 5 days ahead at 10 a.m. ET.