Typically, it costs between $15-35 per car to enter a park that charges an entrance fee. The five fee-free days in 2020 are as follows:
Martin Luther King Jr. Day – January 20
First day of National Park Week – April 18
National Park Service's Birthday – August 25
National Public Lands Day – September 26
Veterans Day – November 11
To see the full list of park sites, by state, you can do so at the NPS website. Keep in mind that even though there are five fee-free days where you can visit at no charge, those entering will still have to pay for extras such as special tours or to camp on-site.