There are two possible sentences..
1) 어떤 해(에) 만나?
= 어떤(which) 해(year) 만나(meet)?
= In which year are we going to meet?
2) 어떻게 만나?
= 어떻게(how) 만나(meet)?
= How are we going to meet?
N.B. the subject "we" I used can be "you" or "they" depending on the context.