Mark it on the calendar, or better yet, write it on your hand: August 25 is a free entrance day at national park sites across the country, and Colorado is one of the best states in the union to take advantage of it. As Westword lays out, the state has 17 national park sites, four of them full-on national parks, and the entrance fees get waived on the 25th for the National Park Service's birthday.

I grew up in Colorado and I'm contractually obligated to be biased, but the bias is earned. The four headliners are Rocky Mountain, Great Sand Dunes, Black Canyon of the Gunnison, and Mesa Verde. That's alpine tundra, the tallest dunes in North America, a canyon that drops 2,000 feet straight down in places, and entire cliff dwellings built into stone by people who figured out the view a thousand years before Instagram did. You could hit one park a season and still not run out.

Quick primer if you're picking one for the 25th. Rocky Mountain is the obvious draw and it's gorgeous, but Trail Ridge Road can still throw afternoon weather at you that'll make you question your life choices. Great Sand Dunes is the one I'd push people toward in late August. Medano Creek season is winding down but the dunes themselves are eternal, and hiking sand at sunset with the Sangre de Cristos behind it is about as good as this planet gets. Bring more water than you think you need. Then bring more.

Black Canyon is the sleeper pick. A lot of people drive to the rim, look down, feel their stomach drop, and leave. Don't do that. Even a short rim walk rewards you. Mesa Verde is the history pick, and a genuinely moving one, though the cliff dwelling tours usually want reservations, so check before you go.

The free day covers entrance fees at the sites that normally charge. It doesn't cover camping, tours, or that post-hike burger, which the universe should really look into. And a heads-up from someone who has done this dance before: free days are popular days. Rocky Mountain runs timed entry in the summer, so a waived fee doesn't get you out of planning. Go early. The crowds at 8 a.m. and the crowds at noon are two entirely different species.

The other 13 sites, things like Bent's Old Fort, Florissant Fossil Beds, and the Sand Creek Massacre site, are quieter and cheaper to begin with, and some are free year-round. Those are the ones where you might have a trail mostly to yourself, which, in late August, is its own kind of gift.

A fee waiver is a good excuse but you never needed one. The outdoors belong to everybody, fee day or not. But I'll take any nudge that gets people through a gate. When's the last time you stood at the base of a 750-foot sand dune? Aug. 25 sounds like a fine day to find out.