Every September (in the 90s) a new cohort of University students would start and gain access to the internet, such as it was. Hence the reference to timing
As the Wikipedia article states, it's more about the specific period of late 1993–early 1994 when some large online service providers in the US started providing access to the greater internet, the USENET in particular, and masses of new (and usually clueless wrt established norms and netiquette) internet users flooded newsgroups and forever changed and disrupted their character and culture.
Yeah, but that was given the name 'Eternal September' as a reference to the literal Septembers of the preceding years, when there would be a regular influx of newbies.
