Viva prep takes more than a day but less than a month.
Prep isn’t knowing everything, but believing you know enough. Read your thesis, make notes, check the literature and find opportunities to practise.
Know who your examiners and what they’ve done – but more importantly know who you are, what you’ve done and what you can do.
You’re not superhuman, but you are superpowered. You can do things that no-one else can.
Prepare for your viva to be able to show this on the day.