~때 : when
~동안 : during, while (certain period of time)
(N.B. sometimes they are interchangeable)
휴가 때 뭐할거에요? = 휴가 동안 뭐할거에요?
(=what are you going to do during vacation?)
집에 갈때 통화해요 (When you go home, talk to me on the phone)
집에 가는 동안 통화해요 (While you are going home, talk to me on the phone (until you reach home))