Pre-debut: K-pop Star, 5LIVE
In 2010, Young K was active under the name Brian Kang as part of a three-member group in Toronto, Canada. Along with members Terry He and Don Lee, he performed at several local stages and has several covers up on Terry He's channel. Along with Don Lee, Young K was contacted by JYP and then auditioned, and then accepted to be trainees. Don Lee later left JYP to pursue medical studies.
In 2012, Jae competed on the first season of SBS K-pop Star. Though he finished in sixth place in the competition overall, he later signed an exclusive contract with JYP Entertainment alongside the season's winner, Park Ji-min and third-place finisher, Baek A-yeon.
JYP Entertainment initially announced the group as a five-member group called 5LIVE, composed of Sungjin, Jae, Young K, Junhyeok, and Wonpil. The group began promotions as 5LIVE in 2014, appearing on the 4th episode of Mnet's reality-survival program Who is Next: WIN and releasing a song titled "Lovely Girl" for the Pretty Man OST. However, the band came to dislike this name as many fans compared their name with that of American band Maroon 5.
In mid-2015, drummer Dowoon was added to the group. In the band's name, the 6 members represent the first 6 days of the week, while the fans represent the 7th day. In preparation for their debut, Day6 performed a series of promotional concerts at live clubs across Seoul.
2015: Debut with The Day, first concert
Day6's debut mini-album The Day, which includes the single "Congratulations," was released on September 7, 2015.
"Congratulations" was also released as a music video on YouTube. Day6 began their first solo concert tour, "DAY6 1st Live Concert," or "D-Day," on November 20 and 21, 2015 at Seoul's Yes24 Movie Hall. Within 5 minutes of tickets going on sale the concert sold out.
2016: Departure of Junhyeok and Daydream
On February 27, 2016, JYP Entertainment stated that Junhyeok has left the band due to personal reasons and terminated his contract with the company.
On March 30, 2016, Day6 returned as five members with their second mini-album Daydream.
cr. Wikipedia