Though many national parks don't charge an admission fee, those that normally do will waive the fee on Sept. 22.
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2018 marks the 25th anniversary of National Public Lands Day, which the National Park Service calls the nation's largest single-day volunteer effort. National parks around the country are offering volunteer work projects, and participants who volunteer on Sept. 22 will receive a free admission coupon to use at a future date.
"(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 general health," the National Park Service said.
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Sept. 22 is the National Park Service's third fee-free day of the year; the fourth and final one is November 11, Veterans Day. Visitors may still be charged for amenity or user fees on free admission days.