(NEXSTAR) — All day Sunday, you’ll be able to visit all of our national parks for free.

Technically, you can visit the majority of the 430 National Park Service sites for free all year long. But on six days in 2024, the entrance fees at the roughly 100 parks that charge them are waived.

We’ve already celebrated three such days this year: Jan. 15, in honor of Martin Luther King, Jr.; April 20, the first day of National Park Week; and June 19, for Juneteenth National Independence Day. After Sunday, there are two more fee-free dates: Sept. 28, to celebrate National Public Lands Day, and Veterans Day on Nov. 11.

If you check your calendar though, you may not see a holiday marking Sunday, Aug. 4. And that’s largely understandable – but for those in the U.S. Department of Interior, it’s a big day.

Sunday will mark four years since the Great American Outdoors Act (GAOA) was signed into law by former President Donald Trump, providing $900 million annually in oil and gas revenues for the Land and Water Conservation Fund, and another $9.5 billion over five years for national park maintenance.

It was the first major financial influx to support NPS infrastructure since 1966, NPS Director Chuck Sams tells Nexstar, adding that it “was desperately needed.”

So far, Sams says more than 600 infrastructure and historic preservation projects have been completed in more than 260 national parks thanks to the GAOA. Roughly 180 of those “are the real large scale, multi-million dollar infrastructure projects,” he adds.

That includes a $17.7 million improvement project at Ellis Island National Museum of Immigration. Next year, officials expect to begin $62 million “critical upgrades” to water and wastewater infrastructure” at Death Valley National Park. More work is on the horizon across NPS.

“We’re working from A to Z,” Sams explains. “Whether it’s at Abraham Lincoln’s birthplace National Historic Park, which has new split rail fencing, or Zion, where we’re rehabilitating the south camp lands campgrounds, we are working extensively and hard all across the system to ensure that these investments that are being made by the American people have many returns on them.”

Sams notes that you may see some of the projects supported by GAOA in progress — they may also interrupt your visit. He recommends “planning like a ranger” by visiting the park’s website before heading out to see what may impact your visit and downloading the NPS app for similar updates and additional details.

If you plan to visit a national park on Sunday to mark the anniversary of GAOA, be sure to note that while admission is free, other amenities and user fees within the park are not waived.