National Public Lands Day is this Saturday, September 26, and that means all national parks in the United States will be free to enter.

In addition to being a free day for park-goers, it’s also one of the biggest volunteer days of the year. The holiday is organized in partnership with the National Environmental Education Foundation, and encourages Americans to join various volunteer projects aimed at preserving the environment, promoting sustainability, and encouraging others to get outdoors. For a list of all the volunteer events in your area, use the National Park Service’s website; there are trail maintenance projects, clean-up events, bird counts, and more.

According to the NPS, National Public Lands Day “celebrates the connection between people and green space in their community, inspires environmental stewardship, and encourages use of open space for education, recreation, and health benefits.”

It’s one of a few days per year that the NPS welcomes visitors to its properties for free, with the next being Veteran’s Day on November 11.