If you're a British woman and invited to the royal wedding, you may be expected to wear a hat. Why?
The BBC says that it is all about royal protocol.
The British social protocol stretches back to before the 1950's when women didn't really show off their hair in public.
The protocol dictates that "women must wear hats to all official occasions."
The royal family is known for keeping traditions that may have faded from the mainstream, like curtsying, to make them stand out.