User blog comment:Randomno/25th Anniversary of the Simpsons - End of Season 23?/@comment-4559601-20111120044342/@comment-961279-20111120070619

Episode 500 is confirmed as part of Season 23 and will air on February 19, 2012. The 25th anniversary would go by date, not episode number. So, if you go by when the first short was seen on The Tracy Ulman Show, the 25th anniversary would be April 19, 2012. For the 25th anniversary of when The Simpsons started as a separate series, that would be December 17, 2014.