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.
It was; AOL in particular, but other ISPs as well.
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.