April 21: First day of National Park Week
Sept. 22: National Public Lands Day
The fee-free days "provide a great opportunity to visit a new place or an old favorite, especially one of the national parks that normally charge an entrance fee," according to the National Parks Service.
Of the 400 national parks in the United States, 118 charge a fee.
The others are free all the time. The waiver on fee-free days does not cover amenity or user fees for activities such as camping, boat launches, transportation, or special tours.
The four free days in 2018 marks a decrease in the number of such days from years past. There were 10 free days in 2017 and 16 in 2016.