[1] On November 21, 2018, News1 reported that Somi was working on new music with the goal of making her solo debut in March 2019.

[9] Ten days later, The Black Label released the third promotional poster of Somi with a mysterious date written on it, confirming her debut the next month.

[15] "Outta My Head" is the other song on the single album, written by Somi, who also participated in its composition alongside 24.

The festive setting was overflowing with popping colors and playful party elements, as if the music video was portraying the song's title in the most vibrant way.

[19][20][21] The lead single of the same name "Birthday" topped major South Korean charts including Mnet, Bugs and Soribada.

It also debuted at the US World Digital Song Sales chart with "Outta My Head" at number 7 and 9, respectively.