All U.S. national parks will be free to visitors on the weekends bookending 2017 National Park Week. That means you can visit any national park for free on April 15, April 16, April 22, and April 23. National Park Week (which is actually nine days) stretches from the 15th through the 23rd.
Over 100 of the 400+ national parks charge an entrance fee (the rest are free), so this is an opportunity to visit a park you might otherwise miss. For example, Yellowstone National Park charges $25 per car, or $15 per person over the age of 16 for those who walk or bike into the park. Grand Canyon National Park is $30 per car or $15 per person.