Haruhi no Happy Birthday Daisakusen is a spin-off episode of the dorama, broadcast in 2011 to promote the upcoming eiga. It was exclusively available on au brand phones through the LISMO website. Four episodes, each five minutes long and broadcast a month apart, tell the story of how Haruhi quits the Host Club due to a birthday misunderstanding. The plot was created by the manga author, Bisco Hatori.
Episode 1: Hikaru and Kaoru overhear two girls talking about Haruhi's "special day" and call the other Hosts to notify them that today is Haruhi's birthday. Meanwhile, the girls find Haruhi and ask to assist with her special day - a big annual sale she's attending. They find her so cute, they log onto the Host Club auction site for some Haruhi items. The boys are planning an extravagant birthday bash when Haruhi appears behind them, angry that they auctioned off her ballpoint pen. Honey suggests they get her pen back from whoever bought it, but Kyoya's records show that it wasn't a fangirl who bought it, but Nekozawa, and the others wonder what need he might have for it.
Episode 2: Kaoru calls Nekozawa to retrieve the pencil, but he refuses to give it back. Angry, Haruhi leaves. Tamaki decides that instead of just “Haruhi’s best 16th birthday” event, they will make it, “Haruhi’s best 16th birthday and apology” event. Mori has to judge which ideas will make the party the "best."
Episode 3: Honey can't resist eating Haruhi's birthday cake.