The "Eternal September" was when AOL got access to Usenet; a regular September was when the students got their shell accounts. The point of the "Eternal" was that the inrush of noobs never went away once AOL was attached.
September was always a rough time on Usenet as all the noobs arrived; some old folks took the month off (or at least claimed to), coming back in October when they'd calmed down a bit. Once the public ISPs arrived it became like September the whole year round.