What do you wear to a 7 PM wedding?

Generally wedding after 7 pm imply formal/cocktail attire. The little black dress would be appropriate and a nice suit to a tux for the men. Take your cues from the formality of the invitation and the location of the reception. If the invite is formal and the location is high end, dress formal. I always follow the rule that I would rather be overdressed and look great than underdressed.