| Earth Day is a somewhat debated holiday. Earthday.net says what I had always heard -- that Earth Day was first celebrated on April 22, 1970, at the instigation of U.S. Senator Gaylord Nelson. His staff member Denis Hayes organized rallies and demonstrations accross the U.S. and brought the threatened condition of the environment to mainstream consciousness. Earthday.net says that "The first Earth Day led to the creation of the United States Environmental Protection Agency and the passage of the Clean Air, Clean Water, and Endangered Species Acts."
However, according to www.earthsite.org, the founder of Earth Day was John McConnell, who intended it to fall on the Spring Equinox (usually March 21). That site says that the April event (which always falls on a Sunday toward the end of the month) was originally called "The Environmental Teach-In" but the name of the equinox event was borrowed by fundraisers for the Teach-In. (Amusingly enough, the writers on earthsite.org do not actually say who is responsible for appropriating McConnell's idea for a different date.)
Nonetheless, the idea of Earth Day, whenever it's celebrated, is to promote environmental action -- people taking care of the Earth, stopping pollution, encouraging conservation of resources, and so forth. | 
