The Autumnal Equinox is when the sun illuminates the Northern and Southern hemispheres equally, also meaning that the day and night hours are approximately equal. The word equinox comes from the Latin 'aequinoctium' meaning equal night.
To celebrate this event (and also claim your autumn equinox souvenir), there will be a gathering at the car park at the top of Wye to watch the sunset together. The meet is arranged for the car park (a voluntary charge applies), so parking is nice and easy. Feel free to bring a chair or blanket and sit on the grass opposite to watch the sunset, which will be just before 7pm.