If the episode aired at 6(pm?), it's reasonable to assume you saw a syndicated episode. Different episodes air in syndication around the country (& world) and since no one has any idea where you are or what channel you saw it, not to mention you provided no information about the episode & the episode aired 14 years ago (as of this answer) this question is impossible to answer.
October 2, 2000 for "Under the Night".
The Simpsons - 1989 26-2 was released on: USA: October 2014
The Simpsons - 1989 Eight Misbehavin' 11-7 was released on: USA: 21 November 1999 UK: 2 January 2000 Germany: 9 October 2000 Hungary: 15 December 2001
Season 19 episode 2. It's called The Homer Of Seville
The Simpsons - 1989 Cape Feare 5-2 was released on: USA: 7 October 1993 UK: 10 October 1993 Hungary: 18 January 1999 Germany: 2 October 1999
The Simpsons - 1989 Bart Stops to Smell the Roosevelts 23-2 was released on: USA: 2 October 2011 Hungary: 28 October 2012
The Simpsons - 1989 It's a Mad Mad Mad Mad Marge 11-21 was released on: USA: 14 May 2000 UK: 8 October 2000 Germany: 5 February 2001 Hungary: 2 February 2002
The episode was promoted with 2 items, Oscar's Obstacles Truck & the USS Tom Clancy
O Fortuna.
"New Kids on the Blecch"1214 CABF12Original Airdate: 2/25/01
Nothing happens in the 3000th episode because The Simpsons will never reach 3000 episodes.The Simpsons only air 22 episodes a season so at that rate it would take 136 years (from its initial debut) to reach 3000 episodes & it's absolutely impossible the show will still be on the air in 2126.
The Simpsons - 1989 Itchy and Scratchy Land 6-4 was released on: USA: 2 October 1994 Germany: 5 August 1995 Hungary: 24 December 2000 Finland: 5 May 2006 (re-release)