On November 15, 2011, Loen announced, that IU was signed to EMI Music Japan and was set to release an extended play with her greatest hits recorded in Japanese, on December 14, 2011, titled I□U.

[1] The Korean version of "Good Day" was written by Kim Eana, while production was handled by Lee Min-soo.

[2] Writing for New Straits Times, Bibi Nurshuhada Ramli described "Good Day" as IU's breakout song.

[4] In November 2019, Billboard magazine compiled a list of the 100 Greatest K-Pop Songs of the 2010s, with the single "Good Day" ranking at number one.

[5] Commercially, "Good Day" debuted at number one of the South Korean singles chart on the 51st issued week of 2010.