Royal Wedding: Proper Guest Attire for Prince William and Kate's Big Day

April 28, 2011— -- There has been endless speculation about what princess-to-be Kate Middleton will wear on the big day.

But what about the guests?

On Friday, 1,900 lucky guests will convene in Westminster Abbey for the royal nuptials .

Determining what to wear to such a high-end, globally watched affair is no easy matter.

Indeed, some have called dressing for the royal wedding a fashion minefield where one wrong step can -- and will -- land an attendee on a worst-dressed list for years to come.

ABC News went to high-end London-based department store Selfridges for help with the matter.

Because the wedding will take place at Westminster Abbey, leading Selfridges stylists said, sleeves for women are a "do" and looking too sexy is a "don't." Exposing shoulders in a sacred house of worship is considered improper.

And while it's important to look proper, it's ill-advised to appear stuffy. Too many frills or too high a collar can be considered fashion-backward.

Watch a special "20/20" today at 8 p.m. ET for a behind-the-scenes look at the life that awaits Kate Middleton, and tune in again at 4 a.m. Friday for ABC News' live coverage of the royal wedding with Diane Sawyer and Barbara Walters.