Earth from space
Image credit: By NASA/GSFC/Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen [Public domain], <a href="https://commons.wikimedia.org/wiki/File:Earth_from_Space.jpg">via Wikimedia Commons</a>

You are allowed—and, indeed, encouraged—to recycle, clean up litter, avoid pollution, plant trees, and otherwise care for the environment every day of the year. But if you want to make a party out of it, with a billion or so of your closest friends, Earth Day is the day to do so. Pro-environment marches, rallies, and other gatherings have taken place on April 22 since 1970, and 2018’s focus is on ending plastic pollution. If you’d like to participate, head on out to your nearest large park, where you almost certainly won’t be alone.