One Day (2AM and 2PM song)

Prior to both groups' debuts, the members of 2AM and 2PM previously starred as JYP Entertainment trainees in a 2008 reality show on Mnet titled Hot Blooded Men [ko], in which the public would vote on which members would eventually debut.^[1] By the end of the training process, the remaining members became a group known as One Day, although the group would eventually get split into the ballad-focused group 2AM and the dance-oriented group 2PM.^[2][3][4]

Following both groups' Japanese-language debuts, the two groups began filming the documentary Beyond the One Day in Japan in April 2012.^[5] The documentary focused on both groups' rise to fame, featuring content such as bonus footage from the JYP Nation in Japan 2011 concert, the first time both groups performed as One Day in Japan.^[6] The film, directed by Shoichi Omichi and distributed by Toho Pictures, premiered on June 7, 2012,^[7][8] before being released for theaters on June 30, 2012.^[9]

The theme song of the film, "One Day", was first used in a trailer for the film released on May 8, 2012.^[10] A ringtone version of the song was released on June 12, 2012.^[11] The song was later released as a single on July 4, 2012, in twelve editions:^[12]

• Regular edition is the CD single with a bonus track.^[13]
• Type A is a CD + DVD,^[14]
• Type B is the CD single with a front cover artwork of 2PM's member Jun. K,^[15]
• Type C is the CD single with a front cover artwork of 2PM's member Nichkhun,^[16]
• Type D is the CD single with a front cover artwork of 2PM's member Taecyeon,^[17]
• Type E is the CD single with a front cover artwork of 2PM's member Wooyoung,^[18]
• Type F is the CD single with a front cover artwork of 2PM's member Junho,^[19]
• Type G is the CD single with a front cover artwork of 2PM's member Chansung,^[20]
• Type H is the CD single with a front cover artwork of 2AM's member Changmin,^[21]
• Type I is the CD single with a front cover artwork of 2AM's member Seulong,^[22]
• Type J is the CD single with a front cover artwork of 2AM's member Jo Kwon,^[23]
• Type K is the CD single with a front cover artwork of 2AM's member Jinwoon,^[24]

"One Day" is an original Japanese song composed by Super Changddai, who previously worked on several songs for both groups.^[11] The B-side track "No Goodbyes" is an original Japanese song composed by 2PM member Jun. K.^[7] "Angel", a CD-only bonus track, is originally a Korean song from 2PM's debut single album Hottest Time of the Day (2008) that was performed live by both groups at the JYP Nation in Japan 2011 concert.^[25][26]