There are over 400 national parks covering over 84 million acres throughout the United States and its territories. The majority of these parks do not charge entrance fees (though parking fees often apply). As for the 124 national parks that do have entrance fees, there are a number of designated "fee free" days each year.
Here are the National Park Service Free Entrance Days in 2019:
January 21 (Martin Luther King Jr. Day)
April 20 (First Day of National Park Week)
August 25 (National Park Service Birthday)
September 28 (National Public Lands Day)
November 11 (Veterans Day)
The parks that normally charge a fee that are free on the above dates are listed at www.nps.gov/findapark/feefreeparksbystate.htm. California parks in this category include:
Plenty of other amazing national treasures are on this list, like Grand Canyon National Park.
And of course there are two national parks in Ventura County that are always worth exploring:
For more information about the National Park Service, visit www.nps.gov.