Never knew B96 once did a best dance hits of all time countdown. I do recall Flashback Weekends, where the mixmasters would play WBMX-era house, disco and other classic dance. There was a time in 1996 where B96 briefly played music from 1992 and earlier due to a transmitter error. It was mentioned in an 08/30/1996 issue of Radio & Records. Speaking of trade magazines on World Radio History, according to the 4/05/1996 issue of Network 40 (a since defunct radio/music trade magazine) Kiss FM in Los Angeles did something similar during the April Fools weekend where they did their Top 300 Dance Songs of all time.
https://www.worldradiohistory.com/Archive-All-Music/Archive-RandR/1990s/1996/R&R-1996-08-30.pdf
https://www.worldradiohistory.com/Archive-All-Music/Archive-Network-40/96/Network-40-1996-04-05.pdf

I remember WGCI doing commercial free mixes all-day Labor Day in 2000. Also, Kiss FM 103.5 once did a 90s flashback night (playing B96-era music) sometime in 2003 (?) (can’t remember the year). Also, I don’t remember if it fell on a holiday.

Q101 during the original alternative era had a few special countdowns:
https://www.rocklists.com/q101.html

Didn’t WLS 890 do a 60s/70s/80s MusicRadio reunion over a decade or so ago? Also, 97.1 the Drive’s A-Z music blocks is well documented.

As for AT40 and Casey Kasem, I’m more familiar with him voicing Shaggy Rogers in reruns of “Scooby Doo”. Many of the AT40 countdowns were way before my time but for some reason, I couldn’t get enough of them when I started listening to them on MixCloud. I can see why many radio junkies hold Casey to such high regards. AT40 and Casey’s Top 40 both also had special programming themseleves.
https://web.archive.org/web/20180924180811/http://oldradioshows.com/ct40/ct40regindex.html
https://web.archive.org/web/20180907084757/http://oldradioshows.com/at40/specindex.html