Fridays, Cartoon Network
Cartoon Network Wiki; Fridays: The Fansite
This started as Cartoon Cartoon Fridays (Cartoon Network Wiki; TV Tropes), from June 1999 to May 2003. It was hosted by characters from various Cartoon Network shows, and aired various Cartoon Network cartoons (aka "Cartoon Cartoons," hence the name of the block). In September 2003, it changed to just Fridays, and would be hosted by live-action people instead of cartoon characters. I only really remembered Tara Sands as a host, but the internet has reminded me that Fridays was originally hosted by a goofy guy named Tommy and a woman named Nzinga, the latter of whom was replaced by Tara in 2005. Tommy and Tara continued hosting until Fridays ended in February 2007. It was replaced by "Fried Dynamite," which began the era that we shall call Action Fridays.
Many of the shows that aired in this block originated a few years earlier, so the block would air repeats as well as first-run episodes of the shows' later seasons. Though there were some shows that debuted in the CCF era. I won't try to differentiate between them, though, and I very well may be mistaken about whether I first saw some of these things on Fridays or not. I'm also sure to be omitting some series I may have seen a bit of, but which were never of much interest to me. Shows I'm fairly sure I saw some of on CCF included Codename: Kids Next Door (2002-2008), a bit of "Courage the Cowardly Dog" (1999-2002), a bit of "Cow & Chicken" (1997-1999), Dexter's Lab (1996-2003; Ego Trip, 1999), Ed, Edd, n' Eddy (1999-2009), "Grim & Evil" (2001-2002; split into The Grim Adventures of Billy & Mandy, 2003-2007, and "Evil Con Carne," 2003-04, with a finale in 2007), Johnny Bravo (1997-2004), The Powerpuff Girls (1998-2005), Samurai Jack (2001-2004), Sheep in the Big City (2000-2002), Time Squad (2001-2003), a very little bit of "Whatever Happened to Robot Jones?" (2002-2003), and maybe other stuff.
After "Cartoon Cartoon Fridays" changed to "Fridays," the block probably continued airing both repeats and new episodes of some of the series that had already been airing on CCF. Some of the shows that debuted during the "Fridays" era included Ben 10 (2005-2008, though some eps premiered on Fridays and some on Saturday mornings), Fosters Home for Imaginary Friends (2004-2009), Hi Hi Puffy AmiYumi (2004-2006), The Life and Times of Juniper Lee (2005-2007), and maybe other stuff. At least a couple of these shows would continue for a little ways into the Action Fridays era.