The two sentences have quite different meanings:
Wann fängt die Party an?
This sentence quite plainly asks when the party starts.
Sagen Sie mir noch Bescheid?
This sentence, however, modifies the above question - it indicates the asker does not necessarily need to know right away. Especially due to the "noch", this sentence means "Will you inform me in time (before the party starts)?"
This means the asker will accept if they do not get a definitive time right away, but what they want right away is at least a confirmation that they will be informed about the starting time in time, by the person being asked.