Earth Day is a day designed to inspire awareness and appreciation for the Earth's environment. It was founded by U.S. Senator Gaylord Nelson as an environmental teach-in held on April 22, 1970 and is celebrated in more than 175 countries every year. Earth Day is celebrated in spring in the Northern Hemisphere and autumn in the Southern Hemisphere. Many communities celebrate Earth Week, an entire week of activities focused on environmental issues. While the first Earth Day was focused entirely on the United States, an organization launched by Denis Hayes—the original national coordinator in 1970—took it international in 1990 and organized events in 141 nations. Earth Day is now observed each year on April 22 in virtually every country on Earth. Earth Day is now coordinated globally by the Earth Day Network. World Environment Day, celebrated on June 5 in a different nation every year, is the principal United Nations environmental observance.
link: Earth Day on Wikipedia
