An equinox is an astronomical event in which the plane of Earth’s equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September.
On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction. To avoid this ambiguity, the word equilux is sometimes used to mean a day in which the durations of light and darkness are equal.
Autumnal Equinox Day is a public holiday in Japan that usually occurs on September 22 or 23, the date of Southward equinox in Japan Standard Time (autumnal equinox can occur on different dates for different timezones). Due to the necessity of recent astronomical measurements, the date of the holiday is not officially declared until February of the previous year. Autumnal Equinox Day became a public holiday in 1948.
In 2021 Autumnal Equinox Day in USA falls on September 22.