America’s Best Idea – the national parks – gets even better this year with several fee-free days at more than 100 national parks that usually charge entrance fees*.
fee-free days this year:
* June 5-6, 2010
* August 14-15, 2010
* September 25, 2010
(Public Lands Day)
* November 11, 2010
*Fee waiver includes: entrance fees, commercial tour fees, and transportation entrance fees. Other fees such as reservation, camping, tours, concession and fees collected by third parties are not included unless stated otherwise.
Disclaimer - I found this info on the National Park Service Website.