Earth Day is celebrated every year on April 22. What started as a U.S.-focused environmental teach-in in 1970 has grown into a global movement involving around one billion people across more than 193 countries, making it the largest secular civic observance in the world.

The very first Earth Day in 1970 drew an estimated 20 million Americans into rallies, marches, and teach-ins. It was a staggering number at the time and signaled that environmental protection had become a mainstream political force almost overnight.

Before April 22 became the standard date, peace activist John McConnell proposed celebrating Earth on March 21, 1970, the spring equinox. His idea was officially supported by UN Secretary-General U Thant.

The now-famous April 22 Earth Day was championed by U.S. Senator Gaylord Nelson, who envisioned a nationwide environmental teach-in inspired by the energy of student antiwar activism.

The name “Earth Day” was not Nelson’s original idea. It was coined by advertising executive Julian Koenig, who liked that April 22 was his birthday and that “Earth Day” rhymed with “birthday.”

A single full-page ad in The New York Times opinion section helped catapult “Earth Day” into the national vocabulary, generating thousands of letters and donations almost immediately.

One of the biggest financial backers of the first Earth Day was not an environmental group but the United Auto Workers, led by labor leader Walter Reuther. His early $2,000 donation was critical to getting the event off the ground.

Organizers have said bluntly that without the UAW’s funding, printing support, and phone coordination, the first Earth Day might have flopped entirely.

The 1969 Santa Barbara oil spill, which released more than 3 million gallons of oil and killed thousands of seabirds and marine animals, directly inspired many activists who later helped launch Earth Day.

President Richard Nixon and First Lady Pat Nixon marked the first Earth Day by planting a tree on the White House lawn, signaling that environmentalism had reached the highest levels of government.

The environmental momentum sparked by the first Earth Day helped lead to landmark U.S. laws like the Clean Air Act, the Clean Water Act, and the Endangered Species Act in the 1970s.

In 1990, original organizer Denis Hayes took Earth Day global, mobilizing 200 million people in 141 countries and transforming it into a truly international event.

Earth Day 2016 was chosen as the signing date for the Paris Agreement, when leaders from 175 nations gathered at the United Nations to formally commit to a global climate accord.

The unofficial Earth Flag features the “Blue Marble” photograph of Earth taken by the crew of Apollo 17, one of the most reproduced images in history.

New York City’s 1970 Earth Day celebration was so massive that Mayor John Lindsay shut down Fifth Avenue to traffic for miles, turning Manhattan into a giant environmental rally.

In Philadelphia, U.S. Senator Edmund Muskie delivered a major Earth Day address in Fairmount Park, reinforcing the event’s bipartisan appeal.

The character Pogo, created by cartoonist Walt Kelly, became an Earth Day icon with the famous line, “We have met the enemy and he is us,” capturing the idea that environmental harm is often self-inflicted.

The 1990 Earth Day International Peace Climb brought together climbers from the United States, the Soviet Union, and China to ascend Mount Everest together, symbolizing environmental cooperation across Cold War divides. During that same climb, participants removed more than two tons of trash from Mount Everest, proving that even the world’s highest peak is not immune to pollution.

In 2001, Google marked Earth Day with its first-ever Earth Day doodle, launching a tradition of environmental-themed homepage art.

The equinox version of Earth Day, celebrated around March 20, is observed at the exact moment the Sun crosses the equator, making day and night nearly equal worldwide. At the United Nations, the equinox Earth Day is traditionally marked by ringing the Japanese Peace Bell, symbolizing planetary unity.

Coincidentally, April 22, 1970, was the 100th anniversary of Vladimir Lenin’s birth, sparking conspiracy theories at the time that Earth Day was secretly a communist plot.

The FBI reportedly monitored some of the 1970 Earth Day demonstrations, reflecting the tense political climate of the era.