Rihanna was the self-nominated party organiser, and had chosen a night at the MGM Grand in Las Vegas to kick things off, with a performance from Cirque du Soleil, before the pair headed to Club XS.
Katy looked more than pleased with her friend's efforts as she posed alongside her and some of the cast of the show.
Both ladies had opted for long sleeve minis for the occasion, with Katy wearing a sparkling black number with coloured spots, while Rihanna wore a white bodycon dress.
Not failing to mark the occassion in suitable style, Katy had finished her look with a 'bride' emblazoned top hat and veil.
This has got us even more excited for the wedding photos!
By Hayley Spencer