Eternal September

(n, abstract). Back in the day, internet use was restricted largely to scientists, college students, and professors. Therefore, these people were the only people operating the Usenet (Forum - mods = Usenet). Every September, the Usenet would see a large influx of new college freshmen who were not familiar with the established standard of Usenet use. They would temporarily ruin the experience for about a month.
Then came the September of 1994.

That year, thanks to AOL opening up the Internet and Usenet to the masses, the influx of noobs did not end. Soon, the new users vastly outnumbered their predecessors, and nonsense and flame wars became the norm. This phenomenon is referred to as "Eternal September", as the uptick of new users has not ended, nor will it end in the foreseeable future.

Refers to the fall of 1994, when AOL let the great unwashed onto Usenet for the first time. Prior to 1994 Usenet experienced annual suckage in September when new college kids got Internet access for the first time and broke things. After the dawn of AOL-Usenet interconnectivity the suckage didn't stop until 2005 when AOL revoked Usenet access from its members. Now refers to any situation in which new users flood a community and destroy it.