It was started in 1970 by Wisconsin Senator Gaylord Nelson, after he noticed people protesting the Vietnam War, but not putting any pressure on government about the damage being done to the planet via contaminants like oil spills, pesticides, and deadly smog.
20 million Americans celebrated the first Earth Day in 1970. It has since grown, and has been celebrated in more than 192 countries by over one billion civic-minded supporters.
That's the name it was given by the United Nations in 2009. Here in the states, we still call it plain old Earth Day.
The Earth Day Anthem was written in 2013 by Indian poet Abhay Kumar, and has since been recorded in all official UN languages.
Denis Hayes organized the first global Earth Day in 1990. 200 million people across 141 different countries participated in the first global Earth Day.