Preparation Pressure
Plan your viva prep with the pressure you need to get it done.
Time and circumstances are not completely within your control. Planning ahead allows you to think about the situation you need to work well.
If you need space to think, time to make mistakes and enough freedom that you can take a day off from viva preparation then give yourself plenty of time in your plan. Pressure won’t help you. That’s your preference.
Alternatively, if you feel like you need a bit of pressure, then still plan but start closer to your viva day. Work more intensively and focus your efforts on the week and days immediately leading up to your viva. That’s your preference.
Whatever you feel you need and whatever you decide to do, don’t confuse stress with pressure. A little pressure might help you to work well, but stress will only provide pain to your preparation plans.