Hi Astara,
For the open day, you just need to bring yourself (in business wear), an up to date CV and a printed picture (any picture, casual is fine, it's so the recruiter can put a face to the CV when they're reviewing your application later in the day).
When I was invited to an open day earlier in the month, I received the following email.
DRESS CODE
Business Attire :
Ladies - Knee length skirt / dress with skin coloured stockings.
Men - Suit & Tie
Should you wish to attend this event, please bring an updated CV along with a photograph. The Open Day will take up a larger part of the day, depending on the number of candidates. If you are successful at this stage, please note that you will be required to attend our selection process that is conducted over the course of the week.
Hope this helps. Enjoy your day and enjoy the waiting (make sure you sit on the front row - it'll save you a couple of hours).