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.