"rules" are different for different times of day, as well as different formalities
(informal, formal, etc.)
In general, you'll want to wear a dress (doesn't need to be full-length unless it's
a very formal wedding) and just about any color goes except for white and
black.
Dos and Don'ts
*Don't wear white because it competes with the bride. There are plenty of other
colors available. This rule may seem outdated, but it's actually just good
etiquette to not compete with the bride on her special day.
*Don't wear black or sequins during the daytime. Anything too sparkly will
make you look overdressed.
*Do wear something feminine and appropriate, out of respect for your hosts.
Clubwear, overtly sexy clothing (strapless, see-through, etc) doesn't belong at a
wedding. If you have to ask if it's appropriate, it probably isn't.
*Don't wear opera-length gloves (to top of arm) with anything but sleeveless or
strapless gowns.
*Do use good judgment if the invitation doesn't specify the formality of the
event. A pastel suit or soft floral dress for daytime or a little black dress for
evening (after 6 p.m.) will take you almost anywhere.
Choose an appropriate wedding guest dress is a lesson every girl should learn.