Does anyone know when Anna and Elsa's official coronation is?

There's no official information of it, but possibly after 1 Jahr anniversary of their film. Maybe at the end of 2014.

Not anytime soon. Some people from Disney sagte there are no current plans for their coronation, because they want to focus on seperate Frozen merchandise right now, before they will add them to Disney Princess line-up. At this point it is even unknown if Elsa will be added after all, since Disney sagte when they were asked about that 'she is the Queen'.