Eternal September or the September that never ended was a cultural phenomenon during a period beginning around late 1993 and early 1994, when Internet Service Providers began offering Usenet access to many new users.
I was there when it happened. At the time we recognized it as the force that was going to (and did) ruin USENET (and IRC). We did not recognize it as the force that would also ruin the entire world.
Is that correlated with students getting out of school for the summer and joining, or a different event? I have noticed a lower quality of conversation in a number of communities recently.