That Time I Got Reincarnated as a Slime: The Slime Diaries

Peace comes at last following the battle against the Orc Lord, and amidst the duties that keep him busy, Rimuru finds himself with a day off; when he tries to enjoy the break, Rimiru can't figure out what to do with himself.

Summer has come to Tempest; when he learns that summers here weren't always this hot, Rimuru begins to fear that the change is his fault; Shuna and Shion have their own unique way to keep cool.

Extreme summer heat is ravaging Tempest. Rimuru and crew visit a lake they've been told lies beyond the forest, and they change into swimsuits to start having fun... but Shion and Shuna seem dissatisfied with Rimuru's swimsuit. Veldora watches the gang from inside Rimuru's body as he plays a game of shogi with Ifrit. As the Storm Dragon, one who has never experienced swimsuits or water play, he struggles to understand what all the excitement is about...

Rimuru decides to throw a summer festival resembling the ones held in Japan; the monsters have never experienced a summer festival, but when they hear that it's a party, they go all-in to prepare.

Rimuru plans celebrations for the Bon Festival.