On April 21, everyone is getting in free to kick off National Park Week, which lasts through April 29. There will be activities throughout the week. It starts with a kick-off for National Volunteer Week on April 21 and Earth Day festivities on April 22, including a celebration of the 50th anniversary of the National Trails System and the Wild & Scenic Rivers System.
Additionally, April 29 is National Park Rx Day. That day's programs are designed by the park system with healthcare and community partners to promote the use of the parks as a part of community and individual health initiatives.
The waived entrance fees do not include any costs for transportation around parks or camping spaces.
These dates are worth writing down because the price of going to many national parks is slated to increase quite a bit in May as part of a Trump administration proposal. (Though, the backlash to that plan has the NPS reconsidering the price hikes, which mainly apply to the nation's most popular parks including the Grand Canyon, Yellowstone, and Yosemite.) As frustrating as that might be for folks who love getting outside, it's still a pretty good deal. One ticket gets you entry for an entire carful of people. There are a variety of passes, but lots of them get you access for an entire week as well.