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. All the monsters of Tempest work together and eventually come up with masks, paper lanterns, and even festival floats. On the day of the festival, everyone in the town enjoys piping hot takoyaki, cold shaved ice, yukata, and a Bon dance. In the end, they fill the skies with fireworks as the festival reaches its climax...
This summer festival was 
my own selfish whim.
كان مهرجان الصّيف هذا نزوتي الأنانيّة
Das Sommerfest wurde
nur für mich veranstaltet.
El festival de verano fue
un capricho mío.
El festival de verano ha sido
un capricho mío.
Ce festival,
je l’ai organisé surtout pour moi.
Questo festival estivo è stato un mio capriccio.
Este festival de verão
foi um capricho meu.
Летний фестиваль —
это мой личный каприз.