A Certain Day (ある一日）is a drama cd that came with volume ２of Kokoro Connect Blu-ray. The story takes place after Heartseed has explained the personality exchange but before the park clean-up.
One morning, Taichi left his house and headed for the train station to Yamaboshi High School l. On his way, a personality exchange occured and he found himself inside a train. Not knowing who he swapped with, he tried to find out by saying something to tell from the voice. However, the train was too noisy and he could not hear his own voice. At the next station, Nakayama Mariko said, "Good morning, Iori" and Taichi understood that he was Nagase. Knowing that, he did not have a peace of mind since on the crowded train, Nakayama was getting very close to him. Nakayama asked him whether Iori did her English homework and asked her if he can show it to her.Nagase(Taichi) answered by saying Nagase probably did it but Yaegashi did it and doesn't know whether Nagase did it. Nakayama noticed that the way Nagase (Taichi) was speaking, was weird and wanted to know what did Yaegashi came up. Nakayama got closer to Nagase (Taichi) but Taichi feeling uncomforrtable asked her to move away. Taichi told himself that he is a girl right now and that it is nothing wrong. Nakayama asked Nagase (Taichi) if he did something bad to Taichi. While not knowing what to do, he dashed out of the train at the next station.
Taichi (Nagase) phoned Nagase (Taichi) to confirm the situation and decied to meet at school. Nagase (Taichi) suddenly felt like she wanted to go to the toilet and could hold it in and rushed into the school to find the toilet. She accidentally went into the men's toilet but luckily no one was inside. She corrected his mistake and begins to question whether he should go to the toilet because he is a girl.. While thinking about it, Nakayama came to the toilet and said that he was holding it in on the train so that's why he rushed out. She said, let's go together, and was forced to go into a stall. Nagase (Taichi) panicked and returned to his own body before anything happeneing.
- All of the drama cd that came with the Blu-ray end with the word day (一日）in the Japanese titles.