The autumn equinox - September 23rd is the time when the Green Man, the vegetation god, begins to weaken and die.
In personal, spiritual terms this means that the material world is fraying and becoming thinner, whilst the spirit worlds come through, come closer and more vivid. Our dreams become more vivid, especially on the first full moon after the equinox, as the spirit worlds press in on us and try to communicate with us.
This will reach a kind of climax on Halloween, the ancient festival of the dead, when the spirits of the dead walk more freely on the surface of the earth than at any other time.
(To get my usual plug in, I also show in The Secret History that the green man peering down from the walls of many of our churches is none other than Osiris, the original vegetation god.)