It was 6 monthes since Muzan was burnt to death...

And Yehyuk burnt to death by sunlight with him, and Tanjiro went insane.

Yehyuk Cha was a demon slayer from South Korea. He saved Rengoku at Mugen train. He couldn't saved Tengen from losing his arm, but he saved him from losing his eye. But he got kidnapped by Douma and turned into a demon when the Entertainment District fight was almost ended.

Yehyuk saved all of the Hashiras. Muichiro fell in coma, though. But, more important is...Yehyuk sacrifised himself. Just to kill Muzan. And now, he is dead.

It was enough to make Tanjiro go insane. He lost one of important people than him.

Giyu: "Tanjiro, stop. Yehyuk sacrifised himself to just defeat Kibutsuji."

Tanjiro: "But, why he had to die...? It could be enough when only I died, isn't it...?" He was crying.