It's been said that the national park system is America's greatest idea. If you have spent any time wandering these great ideas, it's easy to see why they're so revered. Each of the 58 national parks is a beautiful, unique space in the country, from in the bison herds of Yellowstone to the rolling hills of Great Sand Dunes, the incredible trees in Sequoia to the eerie blue of Crater Lake.
Many parks in the US are free every day, but others charge an entrance fee. However, there are a handful of days each year when every park in the country opens its doors so you can get in there and enjoy nature.
Here are all the days in 2018 when you can get into US national parks for free:
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