On July 20, 2016 Star Empire announced through a video teaser the upcoming debut of Nine Muses first four-member unit group called Nine Muses' A, setting the release for August 4 at noon KST with no specific details.

[4] On July 25 it was revealed that the single album will be called Muses Diary, releasing the full track list for a total of four songs.

[6][7][8][9][10][11][12] On July 29, an audio teaser of the single was posted online, revealing the songs for the first time.

[14] Muses Diary was released as a digital download on August 4, 2016 through several music portals.

[17][18] In its fourth week, the single album fell from the Top 20, charting at number 21.