One way to celebrate National Public Lands Day? By visiting one of the nation’s most-celebrated parks.

Even better that day, Sept. 28, at the national parks? Entrance fees will be waived.

National Public Lands Day is next on the National Park Service’s calendar for free entrance days on the year. That will mean drivers saving between $10-$35 for their vehicles across the 400-plus sites protected by the National Park Service.

That includes Rocky Mountain, Great Sand Dunes, Black Canyon of the Gunnison and Mesa Verde national parks in Colorado.

It promises to be a busy Saturday at Rocky Mountain National Park, in the middle of the busy season when elk are bugling and fall colors are turning. Be advised the park’s timed entry reservation system runs through mid-October.

Far less busy: the state’s national monuments. Closest to the Front Range is Florissant Fossil Beds National Monument, home to massive, ancient tree stumps and trails through meadows and forests. For a road trip, look to the canyonlands of Dinosaur National Monument in the state’s northwest corner and Colorado National Monument near Grand Junction.

The day’s fee will also be waived at one of Colorado’s most curious national historic sites: Bent’s Old Fort. East of La Junta, the so-called “Castle on the Plains” replicates the major trading post along the old Santa Fe Trail.

Sept. 28 marks the National Parks Service’s fifth free entrance day of the year for special occasions. The last will be Nov. 11, for Veterans Day.