So they invited about 1000 people, about 800 of them Europeans. And Rebecca said that they received more confirmations than they initially expected, so it is going to be a big wedding!
After the wedding ceremony in the Isaac Cathedral they will host a gala dinner in one of the old palaces (she doesn't disclose which one). I think they will set a white tie dress code with loads of tiaras to come out of the various vaults. (But she doesn't talk about the dress code, so that's just my guess.)
Btw, she said that the the ceremony will probably be televised.