The way you have to think of it is that the Bab is the One who ushered in a whole new religious cycle, bringing an end to the prophetic age. That was the day people were waiting for, the return of God. The Bab was the return of God, a Manifestation of God in His own right, not just One who came to announce Baha’u’llah, like John the Baptist announced Jesus.The big day in the NT is the return of Christ, not any forerunner. Every eye will see him and that day is judgement day etc. And it seems like that should be the day prophesied about. 'Cause isn't the main thing the Bab did was declare that the promised one is coming?
So that big event was the Coming of the Bab, even though His primary mission was to announce the Coming of One greater than Him, Baha’u’llah.