What’s the last thing you will do to get ready for your viva? What final action will leave you satisfied, a small smile on your face that you’ve done all you can?
When will it be? The morning of your viva, a final check of some detail? The day before, deciding what to wear? A few days ahead of your viva when you have a final chat with your supervisor?
Sketching out a plan for prep helps to make it real. Knowing the final step can give you something to guide you. Deciding for yourself what that final task will be really cements that this is for you.
Your research. Your prep. Your viva.