It was released on June 19, 2018, by Oui Entertainment and distributed by Kakao M. After Kim's stint on the second season of Mnet's survival reality series Produce 101 in 2017, he promoted as a member of JBJ with fellow former contestants for seven months.

D-Day peaked at number five on South Korea's national Circle Album Chart and has shifted over 22,000 units domestically since its release.

Kim participated in the second season of Mnet's survival reality series Produce 101 (2017), where contestants competed to become a member of the boy group Wanna One.

[5] Kim performed a cover of Taemin's "Move" at the group's final concert, which led the CEO of his agency to believe he could debut as a soloist.

[6][7] Despite calls from fans for JBJ to extend their period of activities, the members' respective agencies failed to come to an agreement and the sextet disbanded the following April.

[8][9] Kim subsequently began working on solo music in early May and he lost 7 kg (15 lb) within a month in preparation for his debut record.

[15][16] Kim noted his difficulties singing the mini-album's eponymous opening track, describing it as "gentle, bright, and cute" and the "most adorable song" on the record.

[24] A tropical-EDM and chill trap number, "Ain't No Time" expresses the narrator's desire to travel to an uninhabited remote island with a lover.

[26] On June 4, an image teaser was posted across Kim's official social networking service accounts announcing the date of his debut mini-album.

[21] Preceding the release of the lead single "Sunset", a performance trailer was uploaded on June 15, followed by a music video teaser three days later.

[29][30] Kim became the first former JBJ member to release a solo album, and the fourth contestant from the second season of Produce 101 to do so after Samuel, Jeong Se-woon, and Yoo Seon-ho.

[5] Hosted by MC Dingdong, Kim held a showcase for the mini-album at the Shinhan Card Fan Square Live Hall in the Hapjeong-dong neighborhood of Seoul that same day.

[40] He also performed the single and "Ain't No Time" on SBS funE's live concert music program The Stage Big Pleasure.

[41] In support of D-Day, Kim held fan meetings in Thailand, Japan, South Korea, and Taiwan from July to November.