A slight bit of detective work coupled with common sense helps you select wedding attire that's neither too much or too little. When in doubt, don't be afraid to ask for advice from other friends who are attending or even the bride and groom. Ultimately, it's better to be slightly overdressed than to appear as though you didn't care enough to spiff up a bit.
Your invitation might solve your dress dilemma. White tie is the most formal, and for women, that means wearing a ball gown. Black tie calls for a cocktail dress or a longer evening dress, and formal or black tie optional means you should wear a formal evening dress or suit. Typically, the more formal a wedding, the more subdued the color and style of dress should be. If the invitation asks for beach formal, wear a knee or tea-length formal sundress and elegant sandals. When invitations mention casual or dressy casual, err on the side of caution and wear a nice pair of slacks or a skirt along with a classy top.
When the invitation doesn't mention attire, consider the season, venue and time of day. Traditionally, dress is more casual for morning or early afternoon weddings. The later in the day, the more formal events tend to be. Regardless of how formal the event, it's also appropriate to match your attire to the season. For example, a traditional gown in a pastel shade with or without a subdued pattern is ideal for a springtime wedding at a high-class venue.
When in doubt, avoid wearing white, black, or super bright colors. However, these once-taboo colors aren't necessarily frowned upon by everyone. If you think your color choice won't offend but want to soften the look, add splashes of color with a pin, scarf or fitted jacket. Think twice before wearing dresses or skirts that fall well above your knee or have a long slit up the side.
When the invitation doesn't mention attire, and the venue is non-descript, ask someone for an opinion, such as a friend who is also attending. Consider the personality of the wedding couple and whether they typically go for high-end looks or are more laid back. When all else fails, dress conservatively but bring a few items, such as jewelry, a wrap or sash and a fancy hair accessory. This way, if you arrive and notice other guests are dressed more formally, you can ramp-up your look quickly. On the flip side, don't hesitate to tuck a pair of flats or a less formal party dress into your car. In some cases, it's okay to change into more comfortable attire at the reception.