The 2016 Autumn Equonox is September 22nd at 14:21 GMT
English Heritage are expected to offer short period of access, from first light or safe enough to enter the monument field (approximately 06.30am) until 08:30am.
More details as we get them.(source)
The Autumn Equinox (Mabon)
It is the time of the autumn equinox, and the harvest is winding down. The fields are nearly empty, because the crops have been plucked and stored for the coming winter. Mabon is the mid-harvest festival, and it is when we take a few moments to honor the changing seasons, and celebrate the second harvest. On or around September 21st, for many Pagan and Wiccan traditions it is a time of giving thanks for the things we have, whether it is abundant crops or other blessings. It’s a time of plenty, of gratitude, and of sharing our abundance with those less fortunate.
Mabon is a harvest festival, the second of three, that encourages pagans to “reap what they sow,” both literally and figuratively. It is the time when night and day stand equal in duration; thus is it a time to express gratitude, complete projects and honor a moment of balance.
The word ‘equinox’ itself actually mean ‘equal’ (equi) and ‘night’ (nox).