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's 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, including the U.S.
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http://en.wikipedia.org/wiki/Earth_Day
