The Eternal September is a reference to old Usenet groups.

Long story short, Usenet used to be only connected between universities. It had developed its own culture and “nettiquette”. Every September there would be a rush of new users as the new school year began. These new users either learned the rules or they got bored and stopped participating.

Then AOL offered general access to its customers. Suddenly there was a rush of new users, but it was a rush that never stopped. People didn’t bother learning the old culture because there were so many new cultures sprouting up. It, and the rest of the history of the internet, became known as the Eternal September.