“/b/-day” was an outcry on 4chan's /b/ board after rules were imposed on the so-called rule-free board on August 23, 2006 at 2:53pm. It might've marked the end of /b/'s golden age for some, but it didn't actually do much in the end except make some alternative websites more known.

In 2006, 4chan's /b/ board was starting to become known for website raids and false threats, which attracted new dumbfounded users and brought upon the website's first generational strife. Most 4chan users were former Something Awful users, so /b/ exercised its hostility onto furries. By August 2006, they were planning a huge attack on the furry community until 4chan decided to impose stricter moderation on its /b/ board.¹⁾

While they simply reiterated that illegal activities and grey area content were banned, this caused dissident since users believed it meant /b/ was no longer “rule-free”, so they crashed the “img.4chan.org” server for two days and enraged users fled to websites like 7chan, 420chan, AnonIB, Dvach,²⁾ WAKAchan, WTFux, etc. A short war between 4chan and 7chan ensued, then all the “Invasion” (/i/) stuff happened.

• The term is a pun on "D-Day", similar to how everything was an “Operation” back then.

• /b/day on the LURKMORE Wiki