Rest days are important features of a viva prep plan. There’s not so much to do for viva preparation that you have to be working non-stop. It helps to take a break. Allow your mind to consider things from afar and mull over ideas.
Also: just rest.
Take time off from looking at your thesis, thinking about your examiners and wondering what will happen at your viva. It could help to look ahead and plan your rest; tidy up your responsibilities so that you can really take a break.
More than anything, take time for yourself: the knowledgeable, capable and hard-working researcher with their viva in the near future.
Sounds like someone who could use a day off.
