Then why do we observe it only on 22nd of April?
Founded by US senator Gaylord Nelson (an environmental teach-in), the first Earth Day was observed in 1970, on this day, to inspire awareness and appreciation for the Earth's natural environment. In 2009, the United Nations designated April 22 International Mother Earth Day.
With the rising pollution levels and speeding urbanization, over exploitation of natural resources, over-harvesting of forest produces and illegal wildlife trade, the earth is losing its precious life forms and many of its unique biodiversity. The sooner man realizes the value of environment, the better it will be for mankind.
As the famous saying goes:
Only when the last tree has died and the last river been poisoned and the last fish been caught will we realize we cannot eat money.
~Cree Indian Proverb