I went last year. It's basically an on-site employer presentation (like the ones they do on campus usually). They have a few speakers. Someone from HR tells you about the graduate programme, entry requirements, procedures, etc. One of the managers tells you about their career path at Accenture, what they think you need to be a good Analyst, and why they enjoy working there. Then a few of the current graduates tell you about what they have been doing since they joined Accenture, how they found the application process, what they did at university and before they joined Accenture. There are the usual opportunities to ask questions, you watch some promotional videos and you get a few handouts.
We had a stand-up buffet lunch with the people who had been presenting, so you have some chance to ask extra questions and chat informally. You then have a quick tour of the offices. We also did a couple of group exercises. There were about 30 people at the sample day I attended, and we were broken down into groups of about six.
The first exercise was a general knowledge quiz - topics like knowledge of Accenture, current affairs, sports, and I.T. This wasn't a serious test - just one of those "mark yourself" fun things to break the ice with the people in your group. Nobody knew all the answers - the idea is that the spread of questions is so broad that everyone in the group will know some answers but rely on each other for the rest.
The second group exercise was a case study of a sandwich company - we had to discuss and decide on which new business lines the company should explore, and prepare a short presentation of our recommendations. For this exercise, we had a manager there to guide the group as to how to tackle the exercise, manage the time, etc.
Finally, all the groups had to present their recommendations to each other and prizes were awarded for the best presentations. The day was rounded off with tea/coffee and another chance to chat informally with other attendees and people from Accenture.
There is nothing to be worried about on the sample day - you are not assessed or watched at all really. Just look smart and be reasonably polite. I saw some people treating it like an assessment centre, right down to revising their crib notes on Accenture while they were waiting in reception. If you know vaguely anything about the company, you will be going over old ground for most of the first half of the day.
They run the sample day several times so the format and content is quite polished and scripted to match the recruitment website and brochures. The most interesting thing to come out of the question and answers was that apparently a majority of the graduate intake are applying to Accenture for the second or third time i.e. improving at the assessment game with practice.
My view of the day was that it is best treated as an opportunity to decide if Accenture/IT-based consulting is right for you. It's not a close enough simulation of an assessment centre to be worth treating it as practice for the formal application process.
Talk to the other people attending the sample day and compare your impressions and expectations of the job. Also consider whether you could work with those people on a regular basis. In retrospect, the people at the sample day are a fairly good representation of the people who end up working for Accenture.