Traditionally, a six o'clock wedding calls for formal or evening wear. However, many modern wedding parties eschew strict dress policies. Dress code is sometimes noted on the invitation. If there are uncertainties as to what to wear, it is perfectly acceptable to contact the bride or groom and inquire.
According to the Black Tie Guide, formal wear for men means black tie or white tie. For women, an elegant dress or gown is appropriate. Dresses that are low-cut, very short or very flashy (for example, covered in sequins) may be tonally inappropriate for a wedding ceremony. It is considered inappropriate for women other than the bride to wear white at a wedding. In the past, a black dress was associated with mourning and seen as too dour for such a joyful occasion, but in the age of the little black dress, this rule is somewhat outdated and can be discarded.
Your dress choice may also be influenced by the location of the wedding. If the ceremony takes place in a church, it may be advisable to dress more conservatively. In some churches and synagogues, it is appreciated if ladies keep their shoulders completely covered. If your friends are marrying outdoors, consider your footwear choices. Stiletto heels can sink into grass, sand or dirt. Wedge heels, block heels or flats may be a wiser choice for the outdoor wedding.