E! News spoke to royal expert Angela Rippon about the style etiquette, and here's what she spilled about appropriate colors and hat sizes...
Well, according to the official invitation, men should wear a uniform, morning coat or lounge suit. (FYI, folks, a lounge suit isn't a velour tracksuit—it's the typical suit men wear today like these ones Princes William and Harry are sporting.)
But the ladies received no direction about colors or styles on the invite. "There is no code on color at all," Rippon tells us. "There is an unwritten rule that guests don't wear white so they don't upstage the bride." That's an easy one.
However, female guests are expected to wear hats to the ceremony. "[It] probably goes back to the tradition associated with many religions (including Catholic and Greek Orthodox) that women should cover their heads in church," Rippon explains. "The hat is a fashionable answer to the shawl that would have been the obvious and original headgear."
So will any hat do?
Not exactly, Rippon says. "There are no do's and don'ts beyond the obvious, like don't wear a hat that is so large you block the view of others in the Abbey. It will be just a matter of...what goes with the outfit and what suits your face."
Besides hats, many women opt to wear a fascinator—a hair ornament that can be made of feathers, fake flowers or netting, to name a few. "They are very fashionable at the moment," Rippon says, "and Kate wears them quite often herself."
With the clock ticking down quickly, we won't have long to wait to see the guests in all their stylish glory (we're lookin' at you, Posh & Becks!).