Each year, Earth Day is a time to turn our attention toward the environment and raise awareness about threats to its well-being, from air pollution to climate change. But why does Earth Day fall on April 22? Is there any significance to the date?

When the inaugural Earth Day was first suggested by Wisconsin Senator Gaylord Nelson and San Francisco activist John McConnell in 1970, a date was needed that would allow for activists attending college to be free to participate, a point made by organizer Denis Hayes. April 22 fell between spring break and final exams for most universities.

While that was a matter of logistics, April 22 also has a deeper meaning. Earth Day was inspired in part by Arbor Day, a tree-planting event that was organized by Nebraska native Julius Sterling Morton in 1872. Arbor Day was set for April 10. Later, when Arbor Day was declared a legal holiday by Nebraska, the state honored Morton by changing the date to his birthday—April 22.

McConnell actually preferred the Spring Equinox for Earth Day, since marking the changing of the seasons and a balanced amount of daylight and darkness represented Earth’s unique traits. Because April 22 allowed college students to be more active in the events, however, that was the date that stuck.

The modern Earth Day, with its themed objectives and upbeat spirit, is a marked departure from the first Earth Day in 1970, which had demonstrations that bordered on protests. (One enthusiastic group smashed an old Chevrolet to condemn air pollution.) While the world recognizes Earth Day, outside the U.S. it’s actually known by another name: International Mother Earth Day.

A version of this story ran in 2020; it has been updated for 2021.