The first Earth Day took place on April 22, 1970 and was the idea of U.S. Senator Gaylord Nelson of Wisconsin. The event was organized by Denis Hayes, then a graduate student at Harvard University. It was originally planned as a "teach-in" to educate on conservation and environmental issues and helped to create the environmental movement. An estimated 20 million people participated across the country with the largest gatherings taking place in New York City, Chicago, Washington D.C., and Philadelphia. Fifteen hundred colleges and thousands of elementary and secondary schools participated The passage of the landmark Clean Air Act, Clean Water Act, Endangered Species Act and many other groundbreaking environmental laws soon followed. Today it is celebrated in 140 countries with concerts, outdoor events, exhibits, and special features on television and other media.
Source: Lewis, Thomas T. "Earth Day." Encyclopedia of Environmental Issues, Rev. ed. Ed. Craig W. Allin. Vol. 2. Pasadena, CA: Salem Press, 2011. -389. Gale Virtual Reference Library. Web. 2 Feb. 2015.