Al National Park Service sites that charge an entrance fee will offer free admission to everyone. 

National parks are America’s best idea, and there are more than 400 parks available to everyone, every 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. The others are free all the time. The entrance fee waiver for fee-free days does not cover amenity or user fees for activities such as camping, boat launches, transportation, or special tours.