After watching the fireworks with the Armed Detective Agency, it seemed that Atsushi had taken Dazais words.. A bit too hard.

You decide to try and look for Atsushi with Kyōka, as he wasn't in his apartment.

You decide to go to the river where Dazai had found Atsushi at, he was sitting there, waiting for something.. Maybe it was Dazai who he was waiting for.