Eternal September or the September that never ended is Usenet slang for a period beginning in September 1993, the month that Internet service provider America Online began offering Usenet access to its many users, overwhelming the existing culture for online forums. The influx in Usenet users was also indirectly due to the aggressive direct mailing campaign by AOL Chief Marketing Officer Jan Brandt in order to beat out CompuServe and Prodigy, which most notably involved distributing millions of CD-ROMs with free trials of AOL.
Before then, Usenet was largely restricted to colleges and universities. Every September, a large number of incoming freshmen would acquire access to Usenet for the first time, taking time to become accustomed to Usenet's standards of conduct and "netiquette". After a month or so, these new users would either learn to comply with the networks' social norms or tire of using the service.
Whereas the regular September freshman influx would quickly settle down, the influx of new users from AOL did not end, and Usenet's existing culture did not have the capacity to integrate the sheer number of new users following September 1993. Since then, the popularity of the Internet has brought on a constant stream of new users and thus, from the point of view of the pre-1993 Usenet users, the influx of new users in September 1993 never ended.
The term may have first been used by Dave Fischer in a January 26, 1994, post to alt.folklore.computers: "It's moot now. September 1993 will go down in net.history as the September that never ended."
Legacy
In homage to the term, one news server, formerly named Motzarella.org, calls itself Eternal September and gives the date as a running tally of days since September 1993.
A tongue-in-cheek program called sdate outputs the current date, formatted using the Eternal September calendar (September X, 1993, with X an unbounded counter for days since that epoch). This is not the identically named sdate, one of the sixty commands that comes with the First Edition of Unix, that is used to set the system clock.