The equinoctial Earth Day is tied to which astronomical event?

The equinoctial Earth Day is tied to the March equinox.

The March equinox is the astronomical moment when the Sun crosses the celestial equator moving northward. It usually occurs around March 20 and marks the beginning of astronomical spring in the Northern Hemisphere and astronomical autumn in the Southern Hemisphere.

This observance is distinct from the better-known April 22 Earth Day. Peace activist John McConnell proposed a global Earth Day at a 1969 UNESCO conference, and San Francisco held an early proclamation and celebration in March 1970. United Nations ceremonies later continued on the March equinox.

An equinox is a point in time, not necessarily a whole calendar day. Day and night are nearly equal across much of Earth, although the exact balance varies with latitude and atmospheric effects. The March equinox can fall on March 19, 20, or 21 depending on the year and time zone.