EARTH DAY is a name used for two different observances, both held annually during spring in the northern hemisphere, and autumn in the southern hemisphere. These are intended to inspire awareness of and appreciation for the Earth and its environment. The United Nations celebrates Earth Day, which was founded by John McConnell in 1969, each year on the March equinox, while a global observance originated by Gaylord Nelson as an environmental teach-in, and since January 1970 also called Earth Day, is celebrated in many countries each year on April 22.
More information here: https://www.earthday.org/
April 22 was chosen as the date to maximize participation on college campuses for what he conceived as an “environmental teach-in”. He determined the week of April 19–25 was the best bet as it did not fall during exams or spring breaks. Moreover, it did not conflict with religious holidays such as Easter or Passover, and was late enough in spring to have decent weather. More students were likely to be in class, and there would be less competition with other mid-week events—so Wednesday, April 22 was chosen. The day also fell after the anniversary of the birth of noted conservationist John Muir. The National Park Service, John Muir National Historic Site, has a celebration every year on or around Earth Day (April 21, 22 or 23), called Birthday-Earth Day, in recognition of Earth Day and John Muir’s contribution to the collective consciousness of environmentalism and conservation.
Unbeknownst to those making the decision, April 22, 1970, was coincidentally the 100th anniversary of the birth of Vladimir Lenin, when translated to the Gregorian calendar (which the Soviets adopted in 1918). Time Magazine reported that some suspected the date was not a coincidence, but a clue that the event was “a Communist trick”, and quoted a member of the Daughters of the American Revolution as saying, “subversive elements plan to make American children live in an environment that is good for them.” FBI Director, J. Edgar Hoover, may have found the Lenin connection intriguing; it was alleged the FBI conducted surveillance at the 1970 demonstrations. The idea that the date was chosen to celebrate Lenin’s centenary still persists in some quarters, an idea borne out by the similarity with the subbotnik instituted by Lenin in 1920 as days on which people would have to do community service, which typically consisted in removing rubbish from public property and collecting recyclable material. Subbotniks were also imposed on other countries within the compass of Soviet power, including Eastern Europe, and at the height of its power the Soviet Union established a nationwide subbotnik to be celebrated on Lenin’s birthday, April 22, which had been proclaimed a national holiday celebrating communism by Nikita Khrushchev in 1955.