(CNN) -- Here's a look at what you need to know about Earth Day, which is celebrated every year by more than a billion people in 180 nations around the world.
Facts: Earth Day is celebrated every April 22, but some events take place on the weekends before or after the 22nd.
It was created in the United States to increase public awareness of environmental problems and is now celebrated around the world.
Earth Day is credited with starting the environmental movement in the United States.
Timeline: September 1969 - Sen. Gaylord Nelson (D-WI) proposes the idea for a nationwide teach-in about the environment.
April 22, 1970 - The first Earth Day takes place across the United States. Approximately 20 million people participate, taking to the streets and parks to demonstrate for environmental causes.
1970 - Congress creates the Environmental Protection Agency and passes the Clean Air Act.
1990 - The twentieth anniversary of Earth Day sparks renewed interest and an increase in activities and participation.
1995 - Sen. Nelson is awarded the Presidential Medal of Freedom for his role as Earth Day founder and his work on other environmental issues.
April 22, 2007 - Activists rally on Capitol Hill for drastic cuts in greenhouse gas emissions - 80% below 1990 levels by 2050.
March 23, 2010 - Congress introduces a resolution commemorating the 40th anniversary of Earth Day and honoring its founder.