Between 21 September and 24 September
As the earth orbits the sun, there are two days in the year where the day and night are exactly the same length, known as equinoxes. For centuries, dating back beyond the Roman invasion and Christianisation of Britain, these have been celebrated. The second occurs in September, and as Britain is in the northern hemisphere, it is known as the Autumnal Equinox.
The points where the day and night met the same length were as important as the longest day, and the longest night. Whereas the Summer and Winter Solstices showed the heights of the seasons, and therefore the turning of them, the Vernal and Autumnal Equinoxes showed the midpoint between each, in Spring and Autumn, and therefore the annual turning of dark to light, or light to dark.
In me thou see’st the twilight of such dayWilliam Shakespeare, Sonnet 73
As after sunset fadeth in the west,
Which by and by black night doth take away,
Death’s second self, that seals up all in rest.
The Autumnal Equinox takes place amid the harvest season, which began in late Summer and continued until the Harvest Festival, which would happen following the Harvest Moon. It was the moment where control of the crops and seasons changed hands, from the Oak King of the Summer to the Holly King of the Winter, who would reign until the Vernal Equinox in the next Spring. Both Kings would wax then wane, as the moon does, and the cyclic year would continue. It would be marked with a feast, though the main focus traditionally would be upon the start and the end of the harvest season.
The Equinox is still important in terms of seasons and calendars, and hold sway over the current Harvest Festival, which is still acknowledged in Britain. It is sometimes referred to as Mabon, a name given to it in the 1970s, or the Feast of Avalon, however historically it was simply called the Equinox.