This was a hell of a year, leaving many of us scrambling to unwind in one of the country’s treasured natural places. If you’re looking to get outside without dropping cash at the entrance station (which can cost as much as $35 per car, depending on the park), you’re in luck. We’ve compiled a list of all the days the national parks are free in 2021, with tips on how you can get involved and celebrate during these park-centric holidays...