Ever since I started checking out 4chan, I've heard references to an event know as /b/day. From what little I've gleamed, it seemed to be some sort of apocalyptic event that caused a lot of old users ("Oldfags") to flee to 7chan. Do any of you know the story, and if you do, could you tell me what in God's name happened?
I know I should probably be posting this on 4chan itself, but people already have, and no one is has been any help. That's why I'm turning to you guys. I'm starting to suspect it may have never happened.

If anything, I bet it's all the child pornography that runs rampant on that part of the site these days. That's what made me stop visiting it.

No idea, but encyclopedia dramatica usually has everything 4chan related. A quick search yielded a probably correct result:
http://encyclopediadramatica.com//b/day

No problem, it's essentially the same as 4chan itself, although with less blatantly illegal content.
First part of the article (tl;dr summary follows):
On August 23, 2006, all hell broke loose in /b/. Things had been escalating for weeks; CP was posted by the bucketful, and rabid attacks on other websites were increasing. The source of what would be known as /b/-day started the day before, with a rush on myg0t, where many lulz and sadness occurred.
Then the levys broke. A small /b/tard splinter group went absolutely nuts on 3 different forums, completely ruinating them without a hope. Teen babies, feeders, and pro-ana nuts were completely obliterated from the Internets. Nothing was left untouched.
This was all combined with an increase of JB/CP related threads. The mods, being of special mental capacity, decided to do the unthinkable: enforce the rules for /b/. The posting of kiddie porn, jail bait, or raid information/requests (which had always been outlawed, just slackingly enforced) resulted in severe penalties. Moot posted the following notice:
The location of the secret Grey Area is actually Germany much to the disdain of /b/tards.
Policy Enforcement
Participating in a thread dealing with illegal content will get you globally banned for two weeks (thread starter is indefinitely banned). This means merely replying to it removes you from this site for a minimum of two weeks. This includes JB and any other "gray area" threads. Posting any piece of personal information or inciting/participating in an invasion of any sort will also get you, and anybody who replied to the thread globally banned. You have the option to either report or ignore (not reply to) threads that break the rules. Thanks!
Note: To clarify, this applies to THREADS breaking the rules, not REPLIES. If somebody breaks the rules in a REPLY the entire thread won't be banned. Again, exploiting this clause to break the rules will get you banned. These rules aren't anything new--we've just adopted a new way of enforcing them.
There is literally a "Ban thread" button now, so it isn't a chore to do. Abide by the rules and your own common sense.
/b/ now had rules. The entire board revolted; /b/tards turned on /b/tards. Everyone turned on moot. Absolute chaos ensued.
In the aftermath:
/b/ and most of 4chan was completely DDoS'ed.
There was a mass exodus of /b/tards, most likely to never come back
4chan will completely fucking utterly suck forever now, and lose most of its users.
Finally proved the statement "Only /b/ can destroy /b/".
Initially an obscure imageboard, 7chan became a new home to many exiled /b/tards and has risen to become one of the biggest chans on the internets.
And nothing of value was lost.
PROTIP: Banning DESERVING call-to-arms for raids was an absolute crock, as it is and has always been truly EPIC raids that united/still unite us moar than anything else, and give /b/ our historical aspect on teh intarwebs.
tl;dr:
moot made it so they could ban people who posted CP easily (and those who posted in the threads). Forum-goers QQed and went to 7chan.

Encyclopedia Dramatica: "We wish we were Uncyclopedia."

That's it?! That's the doomsday event that I've heard whispers of? Jesus, all that happened was the enforcing of rules. I can't believe people freaked out just because The mods did their jobs! I understand it's a bit deeper than that, with the whole rampant posting of CP, but still, all that happened was the rules got enforced.

Ugh, Uncyclopedia is a giant piece of shit. That place is unfunny beyond relief.

I start to reconsider visiting some of the boards.
The last spam attack was creeping users really out. Because the virus were within the image files uploaded and could only be activated via renaming them to ".hta". The virus itself wasn't that bad, it just caused your comp to run a script to upload spam posts to fuck the 4chan server up, but that's bad enough for many who aren't dumb /b/tards.
Some suggest the infamous Kimmo Alm was behind this. Anyway, you should be careful from there on. Some idiots got the idea making the virus activating as .jpgs or so they posted. Not sure if ti would work, but you never know...
Aside from that, spam got reduced through permab& and now can enjoy funny batman threads:
http://boards.4chan.org/co/res/18734761

There was a proof-of-concept for jpeg "viruses" back in 2002, but it required a program already be installed on a target machine that would then execute code appended to jpeg files (so basically a machine already had to be infected before an additional payload could be delivered via a jpeg). The issue with delivering viruses via jpeg (or other common image formats) is that programs do not treat the data contained in such image formats as executable code, and without programs being willing to execute code packing a virus into an image is simply a non-starter. Of course, you'll still have the occasional script kiddies trying the old "virus.jpeg.exe" trick to fool naive people into executing what they think is just an image file.

Ugh 4Chan is trash

100% of 4chan users are morons
If your metal age is above 12 then you have no reason to visit the site.

No need to apologise for not wanting to visit a malware riddled site that abandoned its true purpose a long time ago and descended back into the cesspit of its own creation.

I don't go to 4chan anymore. I just wanted to see what it was, and now that I've had my fill, I'm done. I heard about the .hta, though. Pretty scary shit, especially if what DarkPhoenix said is true. /b/ truly is the Hell of the internet.

Yeah I went to it like a year ago, but the moment I got a pop-up I freaked and closed the browser. I'm a little paranoid like that. What was it's "true purpose?" I just assumed it's always been a POS.

You've got it backwards. Wikipedia is supposed to be the parody of Uncyclopedia, not the other way around. Uncyclopedia has tons of useful articles, such as how-to articles on cutting off your head with a chain saw or a circle saw.