Buckingham Palace Christmas Diplomatic Reception — December
Women: The "white tie" dress code is the fanciest one can get. For women, it means a floor-length gown and tiara. If they have any honors, they will wear them.
Men: White tie, dress shirt, and tailcoat. Particular to the Knights of the Garter — including Prince Philip, Prince Charles, and Prince William — is the additional regalia that goes with it: the Garter Riband, Garter Star, and Garter itself.