From Wikipedia:

Earth Day is a name used for two different observances, both held annually during spring in the northern hemisphere. These are intended to inspire awareness of and appreciation for the Earth's environment. The United Nations celebrates Earth Day each year on the vernal (March) equinox; while a global observance in many countries is held each year on April 22.

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Now observed in 175 countries, and coordinated by the nonprofit Earth Day Network, according to whom Earth Day is now "the largest secular holiday in the world, celebrated by more than a half billion people every year." Environmental groups have sought to make Earth Day into a day of action which changes human behavior and provokes policy changes.

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