The line between formal and less formal weddings is often drawn at the 6 p.m. line. Weddings held at 6 p.m. or later are considered formal, so evening wear is appropriate. However, you can take cues from elements on the invitation to decide on an appropriate outfit.
Definition of White Tie
Some invitations may specify the wedding is black tie, black tie optional or white tie. White tie is considered the most formal of weddings. For women, long dresses or ball gowns would be appropriate, and for men, a black tuxedo (possibly with tailcoat) with a white tie or bow tie is appropriate.
Definition of Black Tie
Black tie means men should wear a tuxedo and women should wear a cocktail dress or an understated long dress. The dress can be black if the wedding is after 6 p.m.
Black tie optional means men may wear a tuxedo, but a dark suit would also be appropriate. Women may wear the same as they would for a black tie wedding.
Invitation
Some invitations may specify the wedding is black tie, black tie optional or white tie. If the invitation does not specify attire but it is extremely formal looking--that is, it has separate response and reception cards, elaborate calligraphy, or just an overall formal appearance, white tie or black tie should be considered.
The Wedding Site
If the wedding is to be held at a ballroom, country club or other grand location, the wedding attire should be formal. If the wedding is outdoors or at someone's home or backyard, a dress is still appropriate, but a linen or polyester blend is advisable.
The Attendants' Attire
If you have a friend who is in the wedding party, ask her what the attendants will be wearing. If the bridesmaids' dresses are long, particularly if they resemble evening gowns or have a wrap, formal black tie or white tie outfits would be appropriate for the guests.
When in Doubt
For women, a black cocktail-length dress is understated and elegant for almost any evening affair. For men, a tuxedo with a black tie should work.