When is Earth Day?

Earth Day is an annual event on April 22 to demonstrate support for environmental protection. First held on April 22, 1970, it now includes a wide range of events coordinated globally through earthday.org (formerly Earth Day Network) including 1 billion people in more than 193 countries. In 1969 at a UNESCO conference in San Francisco, the peace activist John McConnell proposed a day to honor the Earth and the concept of peace, to first be observed on March 21, 1970, the first day of spring in the northern hemisphere. This day of nature's equipoise was later sanctioned in a proclamation written by McConnell and signed by Secretary General U Thant at the United Nations.

From Wikipedia’s article Earth Day, by its contributors, under CC BY-SA 4.0.