This is the 3922nd most frequent Korean word.
예정이었던
"That was scheduled" or "that had been planned."
The word '예정이었던' is used here to describe the flight that was originally planned or scheduled.
우리가 예정이었던 비행이 연기되었습니다.
The flight we were scheduled for has been delayed.
The word '예정이었던' explains that the presentation had been planned to happen.
그가 예정이었던 발표를 취소해야 했습니다.
He had to cancel the presentation he was scheduled to give.
Here, '예정이었던' refers to the event that was scheduled to take place this weekend.