For those of you who aren't keeping tabs on them, DAY6 is a JYP Entertainment group that will release a monthly digital single every month of 2017. Here are their previous releases (After the cut):
JANUARY - I Wait (아 왜)
FEBRUARY - You Were Beautiful (예뻤어)
This video is connected to their debut track, Congratulations, and it features the same actors.
Sources: How Can I Say, I Wait, You Were Beautiful, Congratulations