Earth Day is one of two different observances, both held annually during spring in the northern hemisphere, and autumn in the southern hemisphere. These are intended to inspire awareness of and appreciation for the Earth's environment. The United Nations celebrates an Earth Day each year on the March equinox, a tradition which was founded by peace activist John McConnell in 1969. A second Earth Day, which was founded by U.S. politician Gaylord Nelson as an environmental teach-in in the late 1960s, is celebrated in many countries each year on April 22.

In September 1969, at a conference in Seattle, Washington, U.S. Senator Gaylord Nelson of Wisconsin announced that in the spring of 1970 there would be a nationwide grassroots demonstration on the environment. Senator Nelson first proposed the nationwide environmental protest to thrust the environment onto the national agenda.” "It was a gamble," he recalls, "but it worked."

Five months before the first April 22 Earth Day, on Sunday, November 30, 1969, The New York Times carried a lengthy article by Gladwin Hill reporting on the rising tide of environmental events::

"Rising concern about the environmental crisis is sweeping the nation's campuses with an intensity that may be on its way to eclipsing student discontent over the war in Vietnam...a national day of observance of environmental problems...is being planned for next spring...when a nationwide environmental 'teach-in'...coordinated from the office of Senator Gaylord Nelson is planned...." Senator Nelson also hired Denis Hayes as the coordinator.

Each year, the April 22 Earth Day marks the anniversary of the birth of the modern environmental movement in 1970. Among other things, 1970 in the United States brought with it the Kent State shootings, the advent of fiber optics, "Bridge over Troubled Water," Apollo 13, the Beatles' last album, the death of Jimi Hendrix, and the meltdown of fuel rods in the Savannah River nuclear plant near Aiken, South Carolina -- an incident not acknowledged for 18 years. At the time, most Americans were consuming leaded gas in massive V8 sedans. Heavy industry released smoke and sludge with little fear of legal consequences or bad press. Air pollution was commonly accepted as the smell of prosperity[citation needed]. Environment was a word that appeared more often in spelling bees than on the evening news. The 1970 Earth Day helped to change many peoples' minds.

On April 22, 20 million Americans took to the streets, parks, and auditoriums to demonstrate for a healthy, sustainable environment. Denis Hayes, the national coordinator, and his youthful staff organized massive coast-to-coast rallies. Thousands of colleges and universities organized protests against the deterioration of the environment. Groups that had been fighting against oil spills, polluting factories and power plants, raw sewage, toxic dumps, pesticides, freeways, the loss of wilderness, and the extinction of wildlife suddenly realized they shared common values.

Mobilizing 200 million people in 141 countries and lifting the status of environmental issues onto the world stage. Earth Day on April 22 in 1990 gave a huge boost to recycling efforts worldwide and helped pave the way for the 1992 United Nations Earth Summit in Rio de Janeiro.

As the millennium approached, Hayes agreed to spearhead another campaign, this time focused on global warming and a push for clean energy. The April 22 Earth Day in 2000 combined the big-picture feistiness of the first Earth Day with the international grassroots activism of Earth Day 1990. For 2000, Earth Day had the Internet to help link activists around the world. By the time April 22 rolled around, 5,000 environmental groups around the world were on board, reaching out to hundreds of millions of people in a record 184 countries. Events varied: A talking drum chain traveled from village to village in Gabon, Africa, for example, while hundreds of thousands of people gathered on the National Mall in Washington, D.C., USA.

Earth Day 2000 sent the message loud and clear that citizens the world 'round wanted quick and decisive action on clean energy. Earth Day 2007 was one of the largest Earth Days to date, with an estimated billion people participating in the activities in thousands of places like Kiev, Ukraine; Caracas, Venezuela; Tuvalu; Manila, Philippines; Togo; Madrid, Spain; London; and New York.

Founded by the organizers of the first April 22 Earth Day in 1970, Earth Day Network promotes environmental citizenship and year round progressive action worldwide. Earth Day Network is a driving force steering environmental awareness around the world. Through Earth Day Network, activists connect change in local, national, and global policies. Earth Day Network’s international network reaches over 17,000 organizations in 174 countries, while the domestic program engages 5,000 groups and over 25,000 educators coordinating millions of community development and environmental protection activities throughout the year. Earth Day is the only event celebrated simultaneously around the globe by people of all backgrounds, faiths and nationalities. More than a half billion people participate in Earth Day Network campaigns every year.

The equinoctial Earth Day is celebrated on the March equinox (around 20 March) to mark the precise moment of astronomical mid-spring in the Northern Hemisphere, and of astronomical mid-autumn in the Southern Hemisphere. An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly "above" the Earth's equator, occurring around March 20 and September 23 each year. Although astronomically they occur at the mid-point of the seasons, in most cultures the equinoxes and solstices are considered to start or separate the seasons.

John McConnell first introduced the idea of a global holiday called "Earth Day" at a UNESCO Conference on the Environment in 1969. The first Earth Day proclamation was issued by San Francisco Mayor Joseph Alioto on March 21, 1970. Celebrations were held in various cities including San Francisco, in Davis, California with a multi-day street party, and elsewhere. UN Secretary-General U Thant supported McConnell's global initiative to celebrate this annual event, and on February 26, 1971, he signed a proclamation to that effect, saying:

May there only be peaceful and cheerful Earth Days to come for our beautiful Spaceship Earth as it continues to spin and circle in frigid space with its warm and fragile cargo of animate life.[2]

Secretary General Waldheim observed Earth Day with similar ceremonies on the March equinox in 1972, and the United Nations Earth Day ceremony has continued each year since on the day of the March equinox (the United Nations also works with organizers of the April 22nd global event). Margaret Mead added her support for the equinox Earth Day, and in 1978 declared:

"EARTH DAY is the first holy day which transcends all national borders, yet preserves all geographical integrities, spans mountains and oceans and time belts, and yet brings people all over the world into one resonating accord, is devoted to the preservation of the harmony in nature and yet draws upon the triumphs of technology, the measurement of time, and instantaneous communication through space.
EARTH DAY draws on astronomical phenomena in a new way – which is also the most ancient way – using the vernal Equinox, the time when the Sun crosses the equator making night and day of equal length in all parts of the Earth. To this point in the annual calendar, EARTH DAY attaches no local or divisive set of symbols, no statement of the truth or superiority of one way of life over another. But the selection of the March Equinox makes planetary observance of a shared event possible, and a flag which shows the Earth as seen from space appropriate." [3]

At the moment of the equinox, it is traditional to observe Earth Day by ringing the Japanese Peace Bell, a bell donated by Japan to the United Nations.[4] Over the years celebrations have occurred in various cities worldwide at the same time as the celebration at the UN. On March 20, 2008, in addition to the ceremony at the United Nations, ceremonies were held in New Zealand, and bells were sounded in California, Vienna, Paris, Lithuania, Tokyo and many other locations. The equinox Earth Day at the UN is organized by the Earth Society Foundation.