A day off in the weeks leading up to your viva is not going to detract from your viva performance.
A day off in the week leading up to your viva doesn’t mean that you’re not taking it seriously.
A day off the day before your viva is possibly the best thing you can do, assuming you’ve already invested time in getting ready.
Rest helps. Unless you can’t sit still, unless you just have to do something, then taking some time to relax, recover, rest, destress and focus on being well is perhaps the most helpful thing you can do to help yourself before the viva.
Take today to rest, if you can.