A statement from an S.M. Entertainment representative on March 2, 2016 revealed that the group had finished filming a music video for the lead single. Teasers of the members for the EP's release were unveiled on the group's official Instagram account on March 10, and S.M. Entertainment later revealed the single's title to be "7월7일 (One of These Nights)" which would be released on March 16. However, ten minutes before its release, the company announced that the single, along with its music video and parent album would be released on March 17 instead "to guarantee a high quality of work". To appease their disappointed fans, the band apologized for the delay on the March 16 broadcast of the radio show 'Good Morning FM', where they also discussed their latest album and single. "One of These Nights" is branded as an R&B ballad with a polished rhythm. Jeff Benjamin of "Fuse" described it as a standard ballad which opens "with strings and piano before evolving into a sweeping, orchestral arrangement" until a trap beat kicks in at the 1:40 mark "to give the strings a modern flavor". On the other hand, Chester Chin of "star2" called it a "haunting, downtrodden ballad". Tamar Herman of "Billboard" emphasized the track's "soaring piano, strings, and tinkling chimes" which sits beneath the group's voices "to build the song up to a dramatic finale". It was composed by Hwang Chan-hee, Andreas Öberg, Maria Marcus and penned by Seo Ji-eum of Jam Factory, in the key of F# minor and a tempo of 134 beat-per-minute. Öberg would later continue to contribute his work for the group's releases. Three other versions of the songs were also available upon its digital release, including the De-Capo version, the Joe Millionaire version and the piano version. Lyrically, it is a song about heartbreak and longing, drawing inspiration from the Korean festival Chilseok from which, stems the tragic folktale about the lovers, Jiknyeo and Gyeonwu who were separated and only allowed to meet once a year, on the seventh day of seventh month of the lunisolar calendar, hence the Korean title of the song that literally translates to "July 7". Member Seulgi described it as a sad song about two people in love who have to separate, like the couple from the original tale.