I have no idea. It looks as if it's missing a verb between "rehearsal" and "again," as well as a question mark at the end. From context I suspect the missing word is "begin" and the asker missed the information the first time it was announced.

I suspect somebody has mis-typed the last word. If the last word were 'begin', rather than 'again', it would make perfect sense: When will tomorrow morning's band rehearsal begin?

if you are asking a question with again, it means you have asked the question before. It means. remind me of the answer. Add a question mark to the end and the word BE before again

I suspect somebody has mis-typed the last word. If the last word were 'begin', rather than 'again', it would make perfect sense: When will tomorrow morning's band rehearsal begin?