Summer Days is an erotic visual novel developed by 0verflow, released on June 23, 2006, for Microsoft Windows and later ported as a DVD game and for the PlayStation Portable (PSP). It is the second installment of the School Days series, succeeding the visual novel of the same name and preceding Cross Days. Unlike the previous titles that exist in the same continuity however, Summer Days is a spin-off of the original story retold from the perspective of Setsuna Kiyoura, a high school student out for summer vacation who finds herself attracted to Makoto Itou, a classmate and a patron of a restaurant she eventually comes to work at. The game retains the anime-like presentation familiar to the franchise, requiring little interaction from users, engaging players through a nonlinear plot they are given opportunities to change, and concluding with an ending specific to the choices made during play.

0verflow announced work on Summer Days in October 2005 and marketed it through a promotional campaign consisting of public screenings, sale of merchandise, and a celebratory event on the day of release. In spite of the game's positive performance during this time however, Summer Days was almost universally panned for its heavily bugged state that prompted a disorganized release of large, frustrating patches and an eventual recall. On August 26, 2011, 0verflow announced that the sale of Summer Days was being discontinued and that work on Shiny Days, a modern remake, was underway. The remake was released on April 27, 2012.

Though not as generous as its predecessor, Summer Days made transitions into other media following the game's release. A comics anthology of the series was published, as was two art books of character illustrations and a light novel. The game's soundtrack was also released by Lantis.

As a visual novel, Summer Days contains extremely minimal gameplay. The game's core onscreen presentation is composed of scenes that are viewed from a mostly third-person perspective. At predetermined intervals, the game brakes, and players are presented with one to two responses or actions relevant to the scene in progress to make, or not make, on behalf of characters. Each selection branches the game's progress up that point in an alternate direction, while also causing the player's love toward a character to blossom, plateau, or diminish, thus providing for a nonlinear storytelling experience. As with it predecessor, being an erotic title, relationships between characters may become sexual; the sex scenes have pixelized censors over genitals (the North America release did not censor the genitals). Each route the game takes does invariably conclude at some point with an ending specific to it, thus, players who wish to watch additional endings, and notably aforementioned sex scenes, will have to play through the game more than once.

Much like the other games in the franchise, Summer Days is unusual in that instead of traditionally static characters with subtitled dialogue, it incorporates motion and voice, making it reminiscent of an animated series. Cinamatics naturally play on their own, and players are afforded the ability to pause, fast-forward, and even skip those they've seen before; sex scenes, additionally, become unlockable from the main menu as they are reached in the game. Progress can be saved at any time and loaded from either the main menu or during play.

Limited edition copies of Summer Days were also bundled with the standalone Flash mini-game Ahoge Battle (アホ毛 バトル, Ahoge Batoru), a whimsical take on School Days ending "Bloody End". Choosing from three difficulties, the player controls Sekai, who attempts to fight off Kotonoha by pressing any two configurable keys in quick session. In the case of the highest difficulty, a third is also used. If the player is able to fend off their opponent long enough to fill their end of a progress bar, the game is won and a tally of how many times each key was pressed is recorded. If the player loses, they are given a chance to continue. In an identical fashion, limited edition copies of Shiny Days are expected to come bundled with Strip Battle Days (ストリップバトルデイズ, Sutorippu Batoru Deizu), a rock-paper-scissors mini-game based around the disrobing of female characters.

Setsuna Kiyoura is a high school student out of school for summer vacation, enjoying the break with family and peers. When her childhood friend Sekai is bedridden with mumps and unable to attend to her part-time waitressing job, Setsuna much to her chagrin, agrees to fill in for her. Though she finds the work almost thoroughly unpleasant, from the revealing uniforms to difficult customers, Setsuna manages to overcome the challenges of the job through the encouragement of friends, particularly that of Makoto Itōu, a classmate she likes.

Set in the same universe as School Days, Summer Days retells the story of the first game had it occurred during the midst of summer vacation instead of at school and from the perspective of another protagonist. The game takes place in a fictional, undisclosed prefecture of Japan that spans a range of cities, particularly a coastline called Haramihama, where the game's restaurant and center of activity, Radish, is established. As such Summer Days shares the same setting with a previous 0verflow game called Summer Radish Vaction!! as well as the same characters of Setsuna and Sekai's mothers.