Nonethewiser
A solar
equinox
 is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator (rather than north or south of the equator). On the day of the equinox, the Sun appears to rise "due east" and set "due west". This occurs twice each year, around 20 March and 23 September.
Exact dates can vary a bit from year to year
.
On the days of the equinoxes, the Earth's axis is perpendicular to the Sun's rays, meaning that all regions on Earth receive about the same number of hours of sunlight. In other words,
night and day are the same length all over the world
. This is the reason it's called an “equinox,” derived from Latin, meaning “equal night.”
However, this is a literal translation and not entirely true. In reality, equinox days don't have exactly 12 hours of daylight and 12 hours of dark. Read more here: Equinox: Almost Equal Day and Night.
In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox.
In the Southern Hemisphere, the reverse is true.
Traditions celebrating equinoxes are present in many countries, cultures and religions.
Here are
 some examples of what autumn/fall equinox means to people around the world.
Since autumn/fall equinox celebrations often take the form of a
harvest festival
, and giving thanks to the Earth for its offerings, I would suggest the following visuals for the stamp.
Inspired by the pagan festival
Mabon
, celebrating exactly that: