Below are the lines from Harry Potter and the Deathly Hallows Part 1 (2010):
Voldemort: Come, we've saved you a seat. You bring news, I trust?
Snape: It will happen Saturday next, at nightfall.
What unspecified event will happen at night on Saturday?
The event is that Harry will be moved from his home address to another address.
The book is a little clearer on the subject.
‘My Lord, the Order of the Phoenix intends to move Harry Potter from his current place of safety on Saturday next, at nightfall.’
The interest around the table sharpened palpably: some stiffened, others fidgeted, all gazing at Snape and Voldemort.
‘Saturday … at nightfall,’ repeated Voldemort. His red eyes fastened upon Snape’s black ones with such intensity that some of the watchers looked away, apparently fearful that they themselves would be scorched by the ferocity of the gaze. Snape, however, looked calmly back into Voldemort’s face and, after a moment or two, Voldemort’s lipless mouth curved into something like a smile.
‘Good. Very good. And this information comes –’
‘From the source we discussed,’ said Snape.
They are talking about when Harry will be moved to an unnamed address, as the protection that his mother gave him when she died for him, goes away, because he turns 17 that night.
