What to wear to a wedding rehearsal dinner depends on the style of the dinner. Some rehearsal dinners are very casual affairs, so you will look out-of-place wearing a suit or semi-formal dress. Others are buttoned up, sit-down affairs at which a sundress or pair of shorts wouldn't be appropriate. If you attend the rehearsal dinner, you'll most likely also attend the rehearsal beforehand, so your outfit should be suitable for both occasions.
Backyard BBQ or Picnic
Even though a picnic or barbecue-themed rehearsal dinner may have a relaxed, casual air to it, that doesn't mean you should show up in your most relaxed, casual clothing. If you're a woman, try wearing a cute dress to the picnic or a jean skirt and cute top. It may get chilly as the night goes on, so bring a cardigan to wear over the dress or top. While you want the outfit to be cute and casual, you also don't want to wear something too outlandish. Remember, there will probably be pictures of the dinner and you don't want to look back at an over-the-top dress and wonder what was going through your mind. Men can wear a polo shirt or a short-sleeved button down shirt with a pair of dark jeans or slacks to a picnic rehearsal dinner. Even though it's a casual dinner, leave the shorts at home for another time. The same goes for sandals.
Dinner at a Casual Restaurant
Guests to a rehearsal dinner at a restaurant should dress a little nicer than they would for a picnic but not so nice that they'll stick out. For women, a shift dress paired with a cardigan will look appropriate. If you'd rather wear pants, pair nice trousers with a button-down shirt or blouse. Basic pumps or ballet flats will work on the feet. The look for men at a restaurant rehearsal dinner is similar to the picnic look, though you should avoid jeans, even a dark shade. Trousers that fit well and are a dark neutral color are ideal. Make the look a little more dressy by replacing the polo with a shirt and tie.
While a lot of couples tend to make the rehearsal dinner a more casual affair, in contrast to the wedding, some will want to have a formal dinner. If that's the case, dress appropriately. If you're a woman, wear a cocktail dress. A simple little black dress will make dressing for the dinner easy, but don't wear the same dress you plan on wearing to the wedding. Ties are a must for a formal dinner for men. Leave the goofy ties at home for the dinner and choose a more conservative style that reflects your taste. Stick with fitted, plain front black pants and a white button-down shirt too.
Keep accessories in mind when dressing for a rehearsal dinner. If you're attending a backyard picnic, a floppy straw hat may be a great accessory for a woman. Sunglasses may be a must if the sun's bright, but take them off when it's time to eat. A string of pearls or other delicate necklace will work for a formal dinner or even a dinner at a casual restaurant.