The definition from Caturday.ca:
"Caturday, held every Saturday, is a 4chan meme that involves the mass-posting of cats/kittens. Though the majority of of these images posted are cute and/or hilarious, there is the occassional 4chaner that will find it funny to spoil a caturday thread with pictures of Torturecat, Crushcat and the like (images of dead cats).
Some of these posts are started with common Caturday catch-phrases like "It's Caturday" and "Post some f***ing cats". "
So I'm not saying I get this community totally, and I wish the captions wouldn't be so crude so often, but Caturday postings can be hysterically funny. I'm guiltily hooked. Like the post of somebody's wrinklied-up Sphynx cat captioned: When 900 years you reach, look as good you will not, hm?
Go see - if you have a tough sense of humor. Don't say I didn't warn ya.