WHAT IS EARTH DAY?
Ever wonder how Earth Day began? The first Earth Day was held on April 22, 1970, when San Francisco activist John McConnell and Wisconsin Senator Gaylord Nelson separately asked Americans to join in a grassroots demonstration. Dealing with dangerously serious issues concerning toxic drinking water, air pollution, and the effects of pesticides, an impressive 20 million Americans—10% of the population—ventured outdoors and protested together.
President Richard Nixon led the nation in creating the Environmental Protection Agency, which followed with successful laws including the Clean Air Act, the Clean Water Act, and the Endangered Species Act. Read more from EarthSky.org.
McConnell originally had chosen the spring equinox (March 20, 1970), but Nelson chose April 22, which ended up becoming the official celebration date. (Given that the date of the spring equinox changes over time, it may have made things more complicated to go with the astronimical event rather than just a calendar date.)