A Certain Day of Summer is a Persona 3 Original Drama CD that contains additional story line in the Persona Series. The CD consists of three parts, plus an introduction. It takes place shortly after S.E.E.S. returns from Yakushima. During the CD, the protagonist is always described as him or you, and has no name given.
- Japan: 25 April, 2007 as a Drama CD
A Certain Day of Summer begins with the protagonist being summoned to The Velvet Room. Igor offers to tell him about memories of past and forgotten events. Igor begins by telling a story about a man who dreamed of becoming a butterfly and enjoying flying freely in the skies. After waking, the man wondered if he dreamt of being the butterfly or if the butterfly was dreaming of being him. Igor explains that this man did not know everything about himself, yet he still awakened from dreams.
The story begins during the heat of summer after S.E.E.S. returns from Yakushima. Shuji Ikutsuki is at the Dorm to ask everyone to take Aegis around town so she could experience things for herself. Akihiko and Mitsuru already have plans, so the protagonist, Yukari, Junpei and Fuuka are going out with her. Before going out on the town, they decide to dress Aegis up. Fuuka offers some of her clothing, but Fuuka is shorter and smaller than Aegis, so they don't fit. Yukari's clothing don't fit quite right either. Mitsuru offers some of her clothing as well, but Aegis says Yukari's are the better fit. Aegis prepares to reveal every difference between their measurements, but Yukari panics and manages to quiet her down before the secrets are revealed.
Yukari, Junpei, the protagonist, and Fuuka all take Aegis to Gekkoukan High School. While on the train, everyone tries to explain exactly what a school is, but it only serves to confuse Aegis. While talking about giving Aegis a tour, Yukari remembers the protagonist arriving as a transfer student. Junpei remembers being surprised that Yukari and the protagonist came to school together at that time. Fuuka remembers a lot of male students discussing the topic on the school message boards. Yukari is embarrassed and doesn't want classmates talking about her at all. Junpei believes it's good that she's so popular, and Aegis takes note of this fact. Junpei comments about how different things are and says encounters can be important and life altering. Aegis comments that first contacts are important, like battles. Yukari asks Junpei about why Aegis acted so strangely on Yakushima. Despite Junpei trying to hide it, Aegis reveals the way Junpei greeted her the first time. Yukari realizes that Junpei was trying to hit on Aegis, and Yukari laughs at him.
Once the group arrives at the school, they visit key locations around the building, such as the classrooms, student council room, and the PA room. Aegis breaks into a cupboard of strange CDs, a reference to one of Elizabeth's requests. Junpei wants to play them, but Yukari and Fuuka are afraid of getting in trouble. As they explore the school they encounter a note left by Bebe in French. Aegis translates and says that it warns against eating an item labeled "2-E" in the refrigerator. The note appeared to be unfinished. Junpei comments about how it must be a joke, but Fuuka looks disturbed. Yukari asks if she's okay, and Fuuka says she's fine.