I think the first time you can book is at 6 and the last is 9 or 9:30. It's pretty much wide open as far as what time you want tho - we've done 6:00 and 8:00 but have seen folks in/out throughout both meals.
It's more a question of how many servers do they have available and them not wanting all 4 tables (?) for each server showing up at 6. If they bring one in at 6, one at 6:15, one at 6:30 and one at 6:45 they can ensure everyone receives a higher level of service (side note: I've never seen a Palo server have 4 tables, I'm just guessing here)