Eternal September or the September that never ended is Usenet slang for a period beginning around 1993 when Internet service providers began offering Usenet access to many new users. The flood of new users overwhelmed the existing culture for online forums and the ability to enforce existing norms. AOL followed with their Usenet gateway service in March 1994, leading to a constant stream of new users. Hence, from the early Usenet point of view, the influx of new users in September 1993 never ended.
That’s not really true, in hindsight.
The real problem was that the tools for enforcing existing norms and protecting forum culture didn’t exist yet. Look around today, though, and you can certainly find forums and boards and other online spaces where a distinct culture exists and its norms are enforced.