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Check the invitation for dress details. Note the time of the dinner. Note the location of the dinner. Consider the theme of the wedding. Determine the appropriate attire based on the rehearsal dinner menu. Call the venue or hosts if you still aren't sure.
The bride and groom or the groom's parents will have sent you an invitation noting the details of the rehearsal dinner. In addition to the time and date of the event, some invitations note the style of dress the hosts wish for guests to wear. Here are some of the different styles of dress you may see listed on a rehearsal dinner invitation:  Black tie. This style is more common for the actual wedding day, but you may see a request for black tie attire on a rehearsal dinner invitation. Men should wear a tuxedo to a black tie event, while women should wear a cocktail dress or long evening gown. Formal or black tie optional. A formal or black tie optional event indicates that the suggested attire is slightly less formal than a black tie event. Men may wear a tuxedo, but could also wear a dark suit and tie. Women may wear a cocktail dress, dressy suit or evening gown. Dress for a "beach formal" event. If the invitation states that the rehearsal dinner is beach formal, be prepared for an elegant event held outdoors. Guests should dress for the elements, including sun and sand. Men may wear a lightweight suit with a linen shirt, or khaki or linen pants with a cotton button-down shirt. Ties typically are not required at beach formal events. Women may wear a tea-length or knee-length sundress. Select appropriate attire for a "semi-formal" or "dressy casual" rehearsal dinner. For men, semi-formal attire indicates a suit and tie. A dark suit is recommended for evening events, while lighter colors may be appropriate for a daytime event. However, as most rehearsal dinners are held in the evening, darker colors are recommended. Women may wear a cocktail dress or a dressy skirt and top to a semi-formal dinner. Dress for a "casual" event. While casual dress to some people may mean jeans and shorts, in most cases that is not appropriate attire for a rehearsal dinner unless specifically noted on the invitation. Men should wear dress pants with a polo or button-down shirt to a casual rehearsal dinner. Women should wear a dress, or skirt or pants with a dressy blouse. If the invitation doesn't state what type of attire to wear, there are other clues to examine. You can determine a lot about how to dress based on the time of day the dinner is taking place. If it's on the earlier side, say 5:00 pm, it's probably more of a happy hour or cocktail hour, which would imply less formal dress than a full dinner beginning at 7 or so.  Some rehearsal meals are not dinners, but lunches or brunches. If this is the case, the attire will probably be slightly less formal. The time alone is not usually enough to give you a clear indication of what to wear, so take other factors into account along with this one. This is probably the most important factor to take into account when you're deciding what to wear. Fine restaurants may have a dress code, requiring men to wear jackets and ties, for example. More casual establishments won't have a dress code, and neither will most outdoor functions. In these cases you'll have to use your own discretion to figure out what's appropriate to wear. The wedding theme can indicate appropriate rehearsal dinner attire. In general, select rehearsal dinner attire similar to the wedding attire, except less elegant. For example, a beach wedding will likely have a relaxed atmosphere at the rehearsal dinner, which would suggest sundresses for women and slacks with polo shirts for men. A very traditional wedding might suggest classic semi-formal attire for the rehearsal dinner, such as suits for men and cocktail dresses for women. If specified on the invitation, the type of food served can indicate the formality of the event. For example, a clambake on the beach may suggest casual attire, while a five-course meal may suggest semi-formal attire. If you're worried there's a dress code and you aren't sure what it is, it's perfectly fine to call either the venue or the host to find out what will be appropriate. There's no need to go into great detail about what you intend to wear; just ask whether the event will be formal, semi-formal, or informal. After that, it's up to you to pick out the right outfit.