It’s not the day you submit, or the day after.
It’s not the first time you read your thesis after submission, or when you start to get ready for a mock viva.
It isn’t even when you really start to plan for submission, or first think about what your examiners might ask.
The first day of viva prep was a long time ago. The first day of your PhD, whenever you started the work that has lead to your thesis. You have been preparing for a long time before you get to the viva: developing your talent, building your knowledge, getting better.
A little extra prep after submission is needed to be ready. Don’t forget though, for your confidence, that you have been preparing for a long time.