The December issue of Lucky Magazine (Jessica Simpson is on the cover) includes a guide to holiday parties. While primarily designed to show us new clothes and shiny baubles we can purchase for such events, it also answers some eternal questions.
What do I wear when the dress code says "festive?"
Does "holiday attire" require an appliqued sweater?
They also give you advice on what to do if you guess wrong. Underdressed? Stain your lips with wine, unbutton a couple at the top of your shirt, and consider leaving your coat on. Overdressed? Wipe off some makeup, loosen your up-do, and remove some bling.
They missed the real answer; the lie.
When in doubt, overdress. If you find yourself uncomfortable in a room of Santa sweaters and jeans, you can tell them about the next party you will drop by that night! Yes, the holiday season is lousy with parties! Create a blingy-er event later in the evening! Or, if you are at a late get-together, you are coming from a swanky drink -and-appetizer reception.
Of course, if you show up in costume for a non-costume party, I really can't help you.