These questions flood my inbox every day, and I'm not surprised. Often, the invite only states something cryptic like "semi-casual", or when you ask the hostess, she responds, "not too fancy." If it's an event you've never attended before, you can belly-ache over showing up either over- or under-dressed. Everyone has been in this situation before.
Well, help is on the way.
This has to be the most brilliant thing I've heard of in a good long while: STYLEFIX at Net-a-Porter! They list outfit suggestions from head to toe for various events ranging from 'High Society' to the basics, from workwear to vacations, cocktail parties to weddings. And if the outfit suggestions still leave you wondering, they have an 800-number where you can call and have your questions answered in person.