September 22-23 (Northern Hemisphere)
The Autumnal Equinox is one of two days in the calendar year when day and night hours are almost equal to one another; it is also the first day of Autumn, or Fall (September 23-24), in the Gregorian calendar.
The Origin of Equinox Celebrations
The annual seasons happen because of two things: 1) a 23.5° tilt of the earth's axis, which makes the earth rotate like a gyroscope, so it points continuously in a fixed direction-toward the area of space near the North Star; and 2) the earth continuously revolves around the sun in an elliptical orbit-once every 365.25 of our solar days. As a result, for half of the year the Southern Hemisphere is more exposed to the sun than the Northern Hemisphere, and for the other half of the year the reverse is true. At noontime in the Northern Hemisphere the sun appears highest in the sky during summer and lowest in the sky during winter. The Autumnal Equinox is almost exactly halfway between the winter and summer solstices, and in the sky the sun appears about midway between its highest and lowest points. It is one of the two times during the year when day and night are almost exactly 12 hours long, and very close to being equal to each other. The other day is the Spring, or Vernal, Equinox, which usually falls on or near the first day of Spring in the Gregorian calendar.
Whereas the first day of Spring is treated as a time of birth and rebirth, the first day of Autumn is the time when things begin to shed, and the growth which was young in the springtime has now reached maturity. Animals begin preparing for winter, tree leaves turn colors, and the air takes on a chill. Most cultures consider this a time for harvesting crops and taking stock of life's fragility. It is also a time when people begin noticing the waning hours of daylight in anticipation for the Winter Solstice. Since this time of year is one of the most temperate, it is also considered a good time for reflection on the meaning of life.
In every culture the Autumnal Equinox signals the return of weather that favors slowing down, introspection, and conservation. Many cultures celebrate the harvest as a means to defeat the starvation brought on by winter. It is also a time closely associated with the dead. Although we know that the dead do not return to life nor do they walk around on the earth at this time of the year, it is a good time for us to remember, or memorialize, freethinkers who are no longer with us. For freethinkers the Autumnal Equinox is a great time to celebrate the lives of those who contributed to the freethought movement, be they atheist, agnostic, humanist, or rationalist. It is also a perfect day to contemplate your own thoughts on the meaning of life and share them with fellow freethinkers.