I was fortunate enough to be invited to the December 14th Assessment Day!
The good news is that you do not need to go over your personal statement or spend time doing extra reading to prepare for the assessment day.
The lecture was around one hour long, perhaps slightly longer, and was on topics which required no prior philosophical knowledge at all. The assessment, the contents of which are likely to be different for each set of attendees, consisted of 4 questions, all around 10 minutes each. The Professor made it absolutely clear that he was assessing our responses based on what we understood from the contents of the lecture, he explicitly said that he would not like us to bring in any knowledge we have gained from additional reading or prior study of the subject and if we did so it would not be credited.
Overall, it was a worthwhile experience although I am not holding out much hope for an offer. The admissions statistics that we were given were very vague, but I think I estimated around a quarter of applicants are invited to an assessment day, after that it depends on the exact course you've applied for and the number of students they will be accepting onto that course in each academic year.