Here's a timeline for Earth Day, taken from history.com...
June 4, 1916: Earth Day founder Gaylord Nelson born in Clear Lake, Wisconsin. Nelson, a World War II veteran, served in the Wisconsin State Senate and as the Badger State's governor before being elected to the U.S. Senate in 1962.
1963: Senator Nelson accompanies President John F. Kennedy on a speaking tour around the U.S. to raise awareness about environmental issues; however, protecting the environment remains a low priority for most politicians and citizens.
1969: Chemical waste released into Cleveland's Cuyahoga River causes it to catch fire. The event becomes symbolic of how industrial pollution is damaging America's natural resources.
1969: Inspired by the "teach-ins" held by Vietnam War protesters on U.S. college campuses, Senator Gaylord Nelson announces the idea for Earth Day, a large-scale, grassroots demonstration against the degradation of America's natural resources.
April 22, 1970: 20 million people participate in inaugural Earth Day activities around the United States.
1970: Environmental advocacy group Natural Resources Defense Council (NRDC) founded. Today, the group has over 1 million members, a staff of over 300 scientists, lawyers and other specialists and offices in New York City, Beijing, Chicago, Los Angeles, San Francisco and Washington, D.C.
December 1970: U.S. Environmental Protection Agency established by President Richard Nixon in order to protect human health and safeguard the natural environment— air, water and land. Before the agency was founded, "the federal government was not structured to make a coordinated attack on the pollutants that harm human health and degrade the environment," according to EPA.gov. Today, the organization, which is based in Washington, D.C., has over 17,000 employees, 10 regional offices and more than 12 labs.
1971: Environmental activist organization Greenpeace founded. Today, the group, which has campaigned against nuclear power, whaling and global warming, among other issues, has offices in 40 countries around the world.
1972: Congress passes the Clean Water Act, which limits pollutants in rivers, lakes and streams.
1973: Congress passes the Endangered Species Act to protect animals and their ecosystems.
1980: After 18 years in the U.S. Senate, during which time he advocated for numerous environmental causes, Gaylord Nelson loses his race for a fourth-term in office. After leaving the Senate, Nelson becomes a counselor for The Wilderness Society, an environmental group.
1990: The 20th anniversary Earth Day celebrations go global, with participants in over 140 countries.
1995: Gaylord Nelson receives the Presidential Medal of Freedom, the highest award given to a civilian, in honor of his environmental work. President Bill Clinton says of Nelson: "As the father of Earth Day, he is the grandfather of all that grew out of that event."
2000: Hundreds of millions of people in 184 countries celebrate the 30th anniversary of Earth Day, with a focus on "clean energy."
July 3, 2005: Earth Day founder Gaylord Nelson dies at age 89. His New York Times obituary notes that in addition to his Earth Day work, Nelson "was a principal sponsor of laws that preserved the 2,000-mile Appalachian Trail, established fuel efficiency standards in automobiles, sought to control damage from strip mining and led to a ban on the insecticide DDT."
April 2007: Capacity crowds attend Green Apple Festival Earth Day events in New York City, San Francisco and Chicago. More than 40,000 people show up for Earth Day festivities at Chicago's Lincoln Park Zoo, setting a single-day attendance record. Earth Day Network members host 10,000 Earth Day events around the world.