Agreedfor me it's when idols pass away in general. it's really sad.
Also what everyone else said above: the passings of Jonghyun, Hara, Sulli, and many more. and the Ladies Code car accident which resulted in the deaths of 2 membersThis isn't really kpop but the Sewol Ferry Disaster
A lot of idols paid tribute to it, like 2NE1:
This event is especially difficult for me to think about because of something that happened in my past.