At submission, a PhD candidate won’t typically know when their viva is going to be. There’s a period when they’ll wait to find out the date and time for their viva. In the UK it’s common for the viva to be anywhere from six weeks to three months after thesis submission.
A candidate might feel pressure to do something just after submission – from nerves, wondering or hoping – but there’s generally not a great need to start preparing immediately. Viva prep is a different kind of work to what one does to get a thesis finished. The wait after submission can be useful as a break or gear change – a boundary between submission and prep.
Perhaps all you need just after submission is to take a moment. Deep breath. Make a few notes. What will you do for your prep? How will you do it? Sketch a simple plan. Then put it into action when you know your viva date.
After submission, generally, you can relax a little. The hardest work is done. You could be busy – it’s unlikely that your PhD is your only responsibility in life – but at least, for a short time, maybe you can put your research to one side and do something to help yourself rest.