Mabon, Autumn equinox, September equinox, usually falls upon September 21st in the Northern Hemisphere, and marks the first day of Fall, and one of the two days where night and day are of equal lengths (Latin: Aequus = equal, Nox = night).
This is the second harvest festival, and is a time of giving thanks, and starting to prepare for winter.
It is also the time of year when many varieties of mushrooms, including psilocybe species are ready to harvest.