Yesterday
If you’ve already submitted your thesis, then yesterday was the best time to start preparing for your viva.
The next best time is today.
But start small. You don’t have to do a lot today. Spend five minutes. Gather together your thesis and some stationery. Figure out how many days until your viva. Sketch a plan of when you can prepare. Make a short list of things that might help. Decide what you’ll do next. Then stop, that’s enough.
Yesterday would have been great, but today is fine to make a start. Later, you can really get to work.