A peace activist, John McConnell in 1969, brought the idea of Earth Day to be celebrated on the first day of Spring every year to the UNESCO conference in San Francisco.
A US senator made it Nation wide and hired Denis Hayes to be the coordinator. Hayes later took it from the US and made it nation wide in the 90’s.
The equinox occurs in the Spring of the Northern hemisphere and Autumn for the Southern hemisphere. It’s where the sun is directly above the Earth’s equator.
Earth Day is celebrated world wide as the first holy day with no limitations on country, beliefs, etc. People all over the globe are urged to live more green. Turn the lights out, recycle, pick up trash, etc.
Now the equinox or Summer Solstice, you can read about in my post FB Post