Unless its changed since I left, this is how the day goes.
When you turn up, you'll be shown to the kitchen where you can relax and meet the other candidates, and someone from recruitment will come and explain how the day works.
Normally the first stage is written tests. You should get two, one tests your ability to do maths and count money and so on, the other tests your english skills and your ability to understand rules. The papers are marked straight away, and those who fail get sent home then and there, but those that pass go through to the next stage, the group exercise.
Normally, you are all put in a room together, and you get given a story. It's usually something along the lines of your boat has sunk and you have to pick which objects are the most important to survive, or the world is about to end and you have to choose from a list which people you would save. You get given a few minutes to make your own rankings, then you all discuss it as a group and try to reach a consensus together. It's a good way of seeing which people are too quiet or timid to get their voice heard, and which people are too dominant. Communication and team working skills are vital so thats what you really need to demonstrate.
Those who pass that, get to go for an interview.