From the official press release:
The Palace of Monaco announces a change in the date of the wedding of HSH Prince Albert II of Monaco with Ms Charlene Wittstock.
This is because a meeting of the International Olympic Committee is scheduled in Durban from 5 to 9 July and the couple wished for the presence of the Olympic family by their sides. The Prince also wanted the Princess's first trip abroad to be to her homeland, South Africa.
This is why it was decided to hold the ceremonies in Monaco on 2 and 3 July 2011.