This is every named day that I came across in the 4chan /b/ 2006-2008 archive.
-CP Sunday - Established April 9 2006 - Established in this thread. Later seen about a hundred threads later when someone said, "Were you not here a few hours ago? It's like it was CP Sunday".
Scaturday - Established April 9 2006
Dogurday - Established April 9 2006
Winger Wednesday - Established April 9 2006 - Text was "is it winger wednesday yet?".
Math Monday - Established April 9 2006 - Text was "Math monday is soon here, you guys prepared?".
Furry/Furfag Friday - Established April 9 2006 - Established in a thread where someone posted furry porn and someone said "IT'S NOT FRIDAY! D:".
SuiseiSunday - Desu. An anime, and the first thread that had it was supposedly the second thread of it's kind. It had many desu-related images that led many to ask for sauce.
Caturday - Established April 9 2006 - Established in this thread. Even though April 9 was a sunday, an emergency Caturday was called in order to counter the furry spammers that were plaguing /b/ on that day.