How could they write a letter that is so ambiguous that you are not sure if you are accepted?
If its called an *interview* weekend ... But you should definitely call them up and ask them directly what to expect and how many people get accepted versus the number interviewed, are the professors really going to be there on the weekend otherwise who is doing the interview, is it one on one interview or by group, etc.... Then what does casual even mean, no tie, or just wear a lab coat and goggles so you look the part.
And of course don't forget your list of questions to interview them with.
Sounds exciting and fun though.
Best of luck!