From what I can tell based on various blog posts — the most authoritative at LinguisticMystic.com, written by a Colorado linguistics student — they may have evolved from a practice called Caturday, in which cat lovers posted photos of their felines with funny captions on Saturdays.
I had heard of Caturday when I wrote the piece, but I’d understood that it was a proto-lolcats practice that was different from the actual lolcats meme.
Lolcats - images of anthropomorphic felines captioned with mis-spelt web-speak ("im in ur bed zleepin" and so on) - made their first appearance on the site, during its regular "Caturday" slot, but went on to dominate blogs and sites the length and breadth of the internet.
November 5 is a very special day for Anonymous, for this year Guy Fawkes Day and Caturday coincide.
2012 — Katherine Losse, The Boy Kings: A Journey into the Heart of the Social Network, Free Press, →ISBN, page 148:
When he and his wife began to have children, they nicknamed them after Internet memes like the lolcat holiday, Caturday.
2012 — Parmy Olson, We Are Anonymous: Inside the Hacker World of LulzSec, Anonymous, and the Global Cyber Insurgency, Little, Brown and Company (2012), →ISBN, unnumbered page:
In 2005, users on /b/ had started encouraging each other to put funny captions under cute cat photos on Saturdays (or what became known as Caturday).