Question: The Simpsons episode that aired on February 19, 2012 entitled "At Long Last Leave" represented what milestone for the show? 

Context:
Good Night (The Simpsons short)
"Good Night" (also known as "Good Night Simpsons") is the first of forty-eight Simpsons shorts that appeared on the variety show "The Tracey Ullman Show".
 It originally aired on the Fox network in the United States on April 19, 1987, during the third episode of "The Tracey Ullman Show" and marks the first appearance of the Simpson family — Homer, Marge, Bart, Lisa, and Maggie — on television.
 After three seasons on Tracey Ullman, the shorts would be adapted into the animated show "The Simpsons".
 "Good Night" has since been aired on the show in the episode "The Simpsons 138th Episode Spectacular" (in its entirety), along with several other Ullman shorts, and is one of the few shorts to ever be released on DVD, being included in the Season 1 DVD set.
At Long Last Leave
"At Long Last Leave" is the fourteenth episode of "The Simpsons"' twenty-third season, and the 500th episode overall of the series.
 In the episode, the Simpsons discover that the inhabitants of Springfield have grown tired of them and have secretly decided to throw them out of the city.
 After being evicted from Springfield, the family members end up in a rugged place without rules and regulations called The Outlands.
 There, they briefly come across their neighbor Julian Assange, who created WikiLeaks.
 Assange, who is in fear of extradition to Sweden and then the United States, guest-starred in the episode as himself and recorded his lines over the phone having been granted asylum in the Ecuadorian embassy in Britain, while waiting for the results of "Assange v Swedish Prosecution Authority".
You Kent Always Say What You Want
"You Kent Always Say What You Want", formerly known as "Kent State Massacre", is the twenty-second episode of "The Simpsons"<nowiki>'</nowiki> eighteenth season.
 It originally aired on the Fox network in the United States on May 20, 2007 as part of the one-hour season finale, alongside the episode "24 Minutes"; a repeat took place on August 19, 2007.
 It was the milestone 400th episode of "The Simpsons" and was written by Tim Long.
 The episode guest starred Ludacris as himself and Maurice LaMarche as the Fox announcer.
 It was the last episode to air prior to "The Simpsons Movie" releasing into theaters on July 27, 2007.
The Itchy &amp; Scratchy &amp; Poochie Show
"The Itchy & Scratchy & Poochie Show" is the fourteenth episode in the eighth season of the American animated television series "The Simpsons".
 It originally aired on the Fox network in the United States on February 9, 1997.
 In the episode, "The Itchy & Scratchy Show" attempts to regain viewers by introducing a new character named Poochie, whose voice is provided by Homer.
 The episode is largely self-referential and satirizes the world of television production, fans of "The Simpsons", and the series itself.
 It was written by David X. Cohen and directed by Steven Dean Moore.
 Alex Rocco is a credited guest voice as Roger Meyers, Jr. for the third and final time (having previously provided the character's voice in "Itchy & Scratchy & Marge" and "The Day the Violence Died"); Phil Hartman also guest stars as Troy McClure.
 Poochie would become a minor recurring character and Comic Book Guy's catchphrase, "Worst episode ever", is introduced in this episode.
 With "The Itchy & Scratchy & Poochie Show", the show's 167th episode, "The Simpsons" surpassed "The Flintstones" in the number of episodes produced for a prime-time animated series.
Little Orphan Millie
"Little Orphan Millie" is the sixth episode of "The Simpsons"<nowiki>'</nowiki> nineteenth season.
 It first aired on the Fox network in the United States on November 11, 2007.
 This episode represents a milestone in Simpsons' history as it sees Kirk and Luann remarry after initially divorcing eleven seasons earlier.
 It was written by Mick Kelly, production assistant to Al Jean, and directed by Lance Kramer.
 During its first broadcast, the episode garnered 10.57 million viewers.
A Star Is Born Again
"A Star Is Born Again" is the 13th episode from "The Simpsons"<nowiki>'</nowiki> fourteenth season.
 It first aired on the Fox network in the United States on March 2, 2003.
 The episode owes much of its plot to "Notting Hill" (1999).
 While that film is about an actress (Julia Roberts) finding happiness with the owner of an independent bookstore, the Simpsons episode features Hollywood movie star Sara Sloane (Marisa Tomei) falling for Ned Flanders after visiting the Leftorium.
Patty and Selma
Patty and Selma Bouvier ( ) are fictional characters in the American animated sitcom "The Simpsons".
 They are identical twins (but with different hairstyles) and are both voiced by Julie Kavner.
 They are Marge Simpson's older twin sisters, who both work at the Springfield Department of Motor Vehicles, and possess a strong dislike for their brother-in-law, Homer Simpson.
 Selma is the elder by two minutes, and longs for male companionship while her sister, Patty, is a lesbian.
 Kavner voices them as characters who "suck the life out of everything".
 Patty and Selma first appeared on the first ever aired Simpsons episode "Simpsons Roasting On An Open Fire", which aired on December 17, 1989.
The Simpsons (season 23)
"The Simpsons"' twenty-third season began airing on Fox on September 25, 2011, and ended May 20, 2012.
 The showrunner for the season was Al Jean, with three episodes ran with Matt Selman, one of those he also wrote himself.
 The show's 500th episode, "At Long Last Leave", aired February 19, 2012.
List of The Simpsons comics
The following is a list of comic book series based on the animated TV show The Simpsons and published by Bongo Comics in the United States.
 The first comic strips based on "The Simpsons" appeared in 1991 in the magazine "Simpsons Illustrated" (not to be confused with the comic publications from 2012 bearing the same name), which was a companion magazine to the show.
 The comic strips were popular and a one-shot comic book entitled "Simpsons Comics and Stories", containing three different stories, was released in 1993 for the fans.
 The book was a success and due to this, the creator of "The Simpsons", Matt Groening, and his companions Bill Morrison, Mike Rote, Steve Vance and Cindy Vance created the publishing company Bongo Comics.
 By the end of 1993, Bongo was publishing four titles: "Simpsons Comics", "Bartman", "Radioactive Man" and "Itchy & Scratchy Comics".
 Since then, many more titles have been published, out of which "Simpsons Comics", "Bart Simpson", "Bart Simpson's Treehouse of Horror", "Simpsons Super Spectacular", Simpsons Summer Shindig, and "Simpsons Winter Wingding".
The Simpsons 138th Episode Spectacular
"The Simpsons 138th Episode Spectacular" is the tenth episode of "The Simpsons"<nowiki>'</nowiki> seventh season.
 It originally aired on the Fox network in the United States on December 3, 1995.
 As the title suggests, it is the 138th episode and the third clip show episode of "The Simpsons", after "" and "Another Simpsons Clip Show".
 While the "138th Episode Spectacular" compiles sequences from episodes throughout the entire series like the previous two, it also shows clips from the original Simpsons shorts from "The Tracey Ullman Show" and other previously unaired material.
 Like the Halloween specials, the episode is considered non-canon and falls outside of the show's regular continuity.

Answer: show's 500th episode