Eternal September

Before the Internet became mainstream in the mid-1990s, Usenet was largely restricted to colleges and universities, meaning that September (the start of the academic year) represented the largest annual influx of new users. Around 1993, commercial ISPs began offering Usenet access to their userbases, leading to a large amount of new posters with little idea of netiquette joining year-round. From the perspective of experienced Usenet users, the influx of new users in September 1993 never ended.

来源：wiktionary