The first Earth Day, held on April 22, 1970, was the brain-child of Wisconsin Senator Gaylord Nelson and activist Denis Hayes. They were inspired to take action by a massive oil spill in Santa Barbara, California, in 1969. With strong backing from Walter Reuther of the United Auto Workers, celebrations and teach-ins took place in 2,000 U.S. colleges and universities, and roughly 10,000 primary and secondary schools. In addition, hundreds of communities throughout the country participated, bringing the total number of participants to 20 million, or about 10% of the U.S. population at the time. With this event as a catalyst, by the end of 1970 the U.S. Environmental Protection Agency (EPA) was created, and new environmental laws were passed, including the National Environmental Education Act, the Occupational Safety and Health Act (which formed OSHA), and the Clean Air Act, followed two years later by the Clean Water Act, and after that the Endangered Species Act and the Federal Insecticide, Fungicide, and Rodenticide Act. In 1990 Earth Day became a world-wide event, and it is now observed in 192 countries. Denis Hayes claims it is, “the largest secular holiday in the world, celebrated by more than a billion people every year.”
To learn more about Earth Day see Wikipedia, the Earth Day website, and the entry for Earth Day from the Encyclopedia of Environment and Society through the Credo Reference database.