Hi, I am also from Italy and I will attend the 9th of july Florence OD. I can answer you some question.. there can be 3 days: first the OD, then the second day the AD and third day, interview. So I think if you are chosen you must stay 3 days BUT in few cases it happened that there were very few people and they made the OD as the AD, so it kept only 2 days. You will see then.
You need to bring your updated CV, a passport photo (but if you are chosen you have to provide 6 passport photos in business attire), a full length photo in business suit, where you smile, all photos to be made on white background.
You dont need your passport for the open day. I don't even have a valid passport! If you are chosen at the final interview, they will give you time to make your passport, no worries!