These are always explicitly detailed on the invitations to royal occasions, and the royal wedding is no exception, specifying “Dress Uniform, Morning Coat or Lounge Suit”. Women should wear formal dresses or suits, and avoid halter necks, spaghetti straps and mini-skirts. Hats are not compulsory, but many women will choose to wear them - don't go completely over the top (as you would at Royal Ascot). The idea is to look smart and elegant, not show