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 installation of School Days line of 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 fellow 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.