In case the video is ambiguous:
Putting together the ′pretty boys′ from different groups, Boy’s Day will include BTOB’s Min Hyuk, NU’EST’s Ren, VIXX’s Hong Bin and A-JAX’s Seung Jin. They will be parodying Girl’s Day from head to toe, including hair and make-up, in order to present their own version of Something.This is the Girl's Day performance they are mimicking:
Finally, here are some side-by-side pics comparing the members of Boy's Day and Girl's Day.
Update: Just noticed the Jaye Kaye has lots of great photos on her terrific tumblr.