I thought they are open on Fridays.

일요일에 여는 줄 알았는데.

ex)

둘리 : I've just arrived at the gym and found that it's closed.

       (운동하러 왔는데;;;; 닫혔네)

영희 : Really? I thought they are open on Fridays.

       (정말?? 금요일에는 문여는줄 알았는데....)

둘리 : I think we should have called them first. 

       (전화부터 했었어야 하는데....)