National Park Week kicks off on April 21. The National Park Service (NPS) isn't fond of burying the lede, so they've made National Parks all across the United States free for everyone on April 21.
There are just four days in 2018 when national parks across the country will open their doors for free. Many parks are free year-round, but those that charge will give you free rein to explore their beauty. However, only entrance fees are waived. Applicable fees for camping or transportation inside the park are still in effect.
It's worth getting out there and taking advantage of the freebie because a Trump Administration proposal from late 2017 has parks across the country slated to undergo a significant fee increase on admissions. In many instances, the new prices will be more than double the old prices. (However, those fee hikes are being reconsidered at the last minute due to backlash. Among the reasons for the vocal frustration from park-goers is that the increased fees could make US National Parks inaccessible to low-income families.)