This is an allusion to the old days, before the internet became a popular phenomenon. It used to be, that every September a bunch of "newbies" (college student who just access to an internet connection for the first time) would log in and make a mess of things. Then, in the late nineties when it really took off, everybody logged in and made a mess of things. This is this the "eternal september." [1]

[1] https://en.wikipedia.org/wiki/Eternal_September