According to the original plan, it was supposed to be completed next month.
at first; initially; originally
Adverbial use meaning 'in the beginning' or 'at the outset', often contrasting with a later change or outcome.
I was against it at first, but now I'm in favor.
Initially, this problem was underestimated.
According to the original schedule, it was supposed to be finished next month.
This project had problems from the very beginning.
At first many people were opposed, but now more people are in favor.