Quote From delta:
Viva's can happen at any time of year, assuming the time and date is fine by all. Undergraduate students have the Summer off but staff usually work all year round.
In late July and August, staff do have a tendency to vanish as they offload their leave they can't take during term times. It is less likely you'll face viva at these times and also late May and early June, when they're invigilating exams.
Ian (Mackem_Beefy)