"Birthday" was written by Teddy, Brother Su, Bekuh Boom and Danny Chung; composed by Teddy, 24, Bekuh Boom and Somi; and arranged by 24 and R.Tee. It is a dance track described as a funky, clap-happy piece of hip-pop confection. The lyrics follow the singer celebrating herself in a refreshing way. "Outta My Head" is the other song on the single album, written by Somi, who also participated in its composition alongside 24. It was arranged by 24 and described as an alt-R&B track.
Background
On November 21, 2018, it was reported that Somi was working on new music with the goal of making her solo debut in March 2019. On February 23, 2019, it was reported that the debut date would be May and that she will be debuting with a song produced by Teddy, also confirming that the recording process was completed and was currently preparing to shoot the accompanying music video. It wasn't confirmed if the release will be a single or an EP. Shortly after, The Black Label confirmed that Somi will debut in May and that the song produced by Teddy was finished, but the format of the release was not confirmed. Two days later, the agency confirmed that Somi will debut on May 1. On April 29, it was reported that her debut date was delayed due to internal issues in her agency and the anticipated date of her debut is now late May. It was also cited that the delay was in order to consider the quality of the album. Shortly after, the agency stated that "her album production is complete but the timing of her debut has not been decided yet." It was also added that the music video was not filmed yet.
Release
"Birthday" was released on June 13, 2019, through several music portals, including Melon and Apple Music.
Music video
The music video was released alongside the song on June 13.
Commercial performance
"Birthday" debuted at number 51 on the Gaon Digital Chart for the week ending June 15, 2019. The following week, it ascended to number 22 on the Gaon Digital Chart, placing at number 8 on the Component Download Chart and at number 79 on the Component Streaming Chart. The lead single of the same name "Birthday" topped major South Korean charts including Mnet, Bugs and Soribada.
