Exposition Party

All the important characters are brought into a room together so they can drop exposition and summations into the mix. This is accomplished through the pretext of a party, frequently a birthday party.

Anime and Manga

 * This trope was used frequently in the manga of Hana Yori Dango. Birthday and various other parties served as pretexts for four separate declarations of love, two disavowals of family, and the introduction of several major plot points.

Film

 * Rob's going-away party in Cloverfield was used to reveal considerable exposition, such as Rob's relationship with Beth and Hud's crush on Marlena.
 * Used and Parodied all over Reservoir Dogs.
 * The beginning of "Swing Kids" has a party where four of the main characters: Peter, Thomas, Arvid, and Otto, meet up with some girls to swing dance.

Literature

 * The impromptu party that the dwarves throw at Bilbo Baggin's place (where they lay out the plans for the upcoming quest) might qualify for this trope. The Rankin-Bass animated version of The Return of the King begins with all of the principal players at a party, relating all of the events of the movie via song and flashback.
 * The closing chapter of The Westing Game consists of several vignettes that are exposition parties revealing what happened to the characters after the mystery was solved.
 * Also invoked and subverted earlier in the story, where one of the characters throws a party that everyone attends in the hopes that they'll learn important clues from each other that will help one of them win the titular game. It doesn't work and everyone leaves disappointed.

Live Action TV

 * In "Endgame", the final episode of Star Trek: Voyager the story begins at a party at Admiral Janeway's home to celebrate the 10th anniversary of Voyager's return to Earth. Each of the surviving major characters makes an appearance to drop important expository information.
 * Most episodes of LA Law begin with the partners meeting to recap/argue about their current cases.
 * Very rarely do they do this at a birthday or any other party though.
 * A dark version of this trope features in the season 2 finale of Robin Hood. Robin's birthday party is the pretext for getting the gang in one room, but the exposition comes about when the party turns out to be a trap, and the characters decide to share their secrets, thinking they will all be dead by morning.
 * This was used in the Firefly episode "Out of Gas" during a dinner/birthday celebration. The writer of this episode, Tim Minear, has talked about how he threw out as many words as he could 'wife', 'whoring', 'shepherd', 'doctor', 'married', 'companion', 'preacher', 'warrant' (that's all I can think of) to introduce the characters, their relationships to one another and what their place is as part of the crew of Serenity.
 * The Stargate SG-1 episode "2010" starts with Carter meeting with her husband and then going to a commemoration service to honor Earth's alliance with an alien race. These two scenes serve to fill the audience in on what has "happened" since the audience will be completely baffled by the sudden jump 10 years into the future.

Theatre

 * The Stephen Sondheim musical Company.
 * Cyrano De Bergerac: Act I Scene II. All the important characters are in scene (except the protagonist) waiting to see a play. Some of them are presented to Christian, and Ligniere talks about the relationship between Roxane, De Guiche and Valvert. The protagonist himself is absent, so everyone talks about him… because he promised to ruin the play they intend to attend.

Web Comics

 * In Holiday Wars, Halloween takes the three main cast members into a room to give them the backstory of the war, which can be seen in this episode.
 * One of the longest-running arcs (in every sense of the phrase) in El Goonish Shive was "Grace's Birthday Party", in which all but one of the major characters had their gender reversed, and considerable exposition and character development occurred. Ironically, in actual time elapsed in-world, it's the shortest arc.
 * Bob and George had an anniversary party that lasted a couple of days, the last of which was an illusion showing roughly what the webcomic would have been were it the hand-drawn series it was originally intended to be.