The National Park Foundation said National Park Week gives people a chance to enjoy some fresh air.
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Camping fees are still in effect for those who want to stay overnight.
There are three other free national park days later this year.
You'll also be able to visit a park for free on August 4 for the anniversary of the Great American Outdoors Act, National Public Lands Day on September 24 and Veterans Day on November 11.
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RELATED: California library cardholders can check out free passes to state parks
California library cardholders can check out free passes to state parks