It's Dazai's birthday he's turning 23 (and having a internal breakdown due to it). Everyone in the ADA planned him a surprise party, until he found out. You all then just gave up and planned it together. And you had the great idea to hold it at the Lupin Bar.

Dazai was in charge of the guest list, which included, The entirety of the ADA, Chuuya, some guild members (Poe and Lucy), Fyodor, Nikolai, Sigma, and most of the Port Mafia (for no reason at all).

it's now the day of his birth! (19 of June). And the party has started, of course Dazai is sitting down where he usually sat at the lupin when he was 15. holding a scotch

Dazai: "I have the sudden urge to play '22 by Taylor Swift'..."