Day by Day is the fourth extended play by South Korean girl group T-ara, released on July 3, 2012, by Core Contents Media.^[1] The album marked the first appearance of the group's eighth member, Areum, and the last appearance of member Ryu Hwa-young. A repackaged version of the EP, titled Mirage, was released on September 4, 2012.^[2] "Sexy Love" was released as the lead single from the Mirage repackage.^[3]

The EP and its repackage produced the singles "Day By Day" and "Sexy Love". Both the album and singles appeared on Gaon and Billboard Korea yearly sales charts for 2012. However, the album's promotion was cut short due to the group's bullying controversy which resulted in Hwayoung's contract termination and dismissal from the group.

On June 13, 2012, Core Contents Media announced that the group would be releasing a new EP entitled Day by Day on July 3.^[4] The EP marked the debut of T-ara's eighth member, Areum. Both the EP and its title track were released on July 3.

"Day by Day" was written by Kim Tae-Hyun and produced by Cho Young-soo and Ahn Young-min, who have all previously worked with T-ara on songs such as "Cry Cry" and "Neo Ttaemune Michyeo". Before its official release, a choreography video in which "Day by Day" can be heard in was leaked onto YouTube in late May 2012.^[5] The music video for "Day by Day" was directed by Cha Eun-Taek in Seoul, South Korea. It was announced on June 12, 2012, as a 20-minute music video drama starring the group themselves in a science-fiction setting;^[6] reminiscent of the movie Mad Max.^[7] A large amount of work was needed to complete the computer graphics featured in the video, which caused its release to be delayed by 13 hours.^[8] The second part of the music video was released in August 2012. The music video was included in "Top 5 Music Videos of the Week" list by Stereogum.^[9]

A repackaged version of the EP, titled Mirage, was released on September 4, 2012. It contained two additional tracks, "Sexy Love" and "Day and Night (Love All)". A Japanese version of "Day by Day" was digitally released on September 26, 2012, and was included as a B-side on their fifth Japanese single, a Japanese version of "Sexy Love".^[10]

In June 2019, Vietnamese singer Quang Ha was accused of plagiarism after netizens noticed similarities between his newest single "Everyone Will Change" and "Day by Day"'s intro. On July 4, the musician held a press conference in which he admitted that the two songs did sound identical at the beginning. He later apologized for not being aware of international music and not noticing the similarities.^[11][12]

"Day by Day" reached number two on the weekly Gaon Digital Chart^[13] and the K-pop Hot 100.^[14] The song ranked among the best-selling singles of the year with nearly 2,150,000 downloads sold in South Korea by the end of 2012.^[15] "Sexy Love" peaked at number three on the Billboard Korea Hot 100 Songs chart and sold nearly 1,550,000 downloads in 2012. The Japanese version of "Day by Day" peaked at 16 on Billboard Japan Hot 100.

Note: Love All is only included on the physical release.^[16][17]