It's an opportunity to visit all of the great spots across California, such as Yosemite or Redwood, or stay close to home to check out Joshua Tree.
The National Park Service is making the free admission offer to mark the first day of National Parks Week.
Also on Saturday, Yosemite is hosting an "Earth Day Festival" at the visitor center from 10 a.m. to 2 p.m.
There will be interactive booths, live music and special junior ranger walks.