On four days in 2018, all United States National Park Service sites that charge an entrance fee will offer free admission to everyone.
January 15: Martin Luther King Jr. Day
April 21: First day of National Park Week
September 22: National Public Lands Day
November 11: Veterans Day
The parks waiving their fees are listed here: https://www.nps.gov/planyourvisit...-state.htm
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.
Note that not all parks charge an entrance fee; many are always free.