In the last year of the WB's existence, it moved Smallville to Thursday, and since then the 8/7c hour has seen by far the least upheaval of any netlet timeslot. Only once has the CW made a move: when they replaced Smallville with a very worthy heir in The Vampire Diaries in 2009. The 9/8c hour has not been quite so steady; after four plus seasons of Supernatural, the network had a new occupant each year for four seasons. Reign broke the streak by staying put in 2014-15, but it's gone for fall 2015.
|2002-03||Family Affair||Do Over||Jamie Kennedy||Off Centre||23|
|2003-04||Steve Harvey's||Jamie Kennedy||What I Like About You||Run of the House||24|
|2004-05||Blue Collar TV||Drew Carey's Green Screen||(repeats)||25|
|2001-02||Friday Night Smackdown!||59|
|2002-03||Friday Night Smackdown!||48|
|2003-04||Friday Night Smackdown!||48|
|2004-05||Friday Night Smackdown!||45|
|2005-06||Everybody Hates Chris||Love, Inc.||Eve||Cuts||30|
|2009-10||The Vampire Diaries||Supernatural||52|
|2010-11||The Vampire Diaries||Nikita||46|
|2011-12||The Vampire Diaries||The Secret Circle||44|
|2012-13||The Vampire Diaries||Beauty and the Beast||44|
|2013-14||The Vampire Diaries||Reign||42|
|2014-15||The Vampire Diaries||Reign||33|
2014-15 Update: The only thing keeping the CW from good news on every weeknight was a pretty disastrous season on what has usually been one of its brightest nights. Aging The Vampire Diaries (which had to face Grey's Anatomy for the first time) fell to a series low Plus by a wide margin. Its sophomore lead-out Reign was down even more, making it the clear weakest link on the net's weeknight sked. The CW will try to hold this night together with the old regime for one more year by moving TVD spin-off The Originals into the 9/8c hour.
Notable midseason replacements:
After beginning its run on Tuesday, Supernatural swapped over to Thursday for the tail end of season one in 2005-06, where it averaged a 38.
See here for more explanations/disclaimers about Schedules Plus.