I can't say exactly what the structure of the day is like, but I can give you a rough idea of the types of activity they've used in the past.
Typically you'd complete 2-3 tasks on the day. A group activity to see how well you work with others - this could be just one generic task e.g working together to build a paper tower on a budget is one I know has been used, or a task specific to the area you have applied for e.g working in a group to work out how you would handle a hypothetical situation in a queue line.
They may ask you to pre-prepare a short talk on a specific topic, usually an interest that you have, however if they do this you are told before the day.
The group tasks are usually followed by individual interviews. The questions in this usually range from the typical ones you would expect through to more customer service/role specific questions.
I hope that gives you some idea of what to expect. The most important thing is to be yourself and try to relax as much as possible (although I know it's difficult in an assessment!). Good luck
