[@YamiCuoreLaroux]

    There's a gala for the prince(ss)'s birthday and they're all invited for different reasons i.e chef, nobleman, barmaid, servants, etc. 

  This way there's an opportunity for any kind of person to be there, really, unless they're a jobless peasant. It gives the killer the chance to scope out their targets, and maybe some disaster happens at the gala that marks them for death?