Kate looked stunning in a cream jacket (complete with Peter Pan collar) and pleated skirt, pillbox hat, boxy clutch and those patent nude L.K Bennett heels that she just can't part with.
Prime Minister David Cameron, Irish president Michael D Higgins, the King and Queen of Belgium will also attend - so be prepared to see more pristine looks from the duchess.
Will she re-style one of her favourite designer outfits or showcase something new? We'll keep you posted.
LOVE THESE ROYALS? You'll love this gallery of photos all about Kate and Wills.
MORE! Watch all the latest on the Duke and Duchess of Cambridge right here.
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