(CNN) -- There, glaring in red and green at the bottom of the holiday party Evite, lurks the season's most vague directive: "Dress festively."
Even Lizzie Post, the co-author "Emily Post's Etiquette, 18th Edition" and great-great-granddaughter of American manners doyenne Emily Post, doesn't know what that means.
"Festive" clothing can be everything from blinking jewelry and "ugly" Christmas sweaters to a formal gown in a traditional holiday hue. Although Post offers an attire guide for nearly every party situation -- even specifically addressing "festive" -- she readily admits it's a subjective idea.
"When it first started appearing on invitations we were like, 'Oh, crud. That is so open to interpretation. What will people end up wearing?" she said.
The simple answer? Call the party hosts and ask what they envisioned, she said. But if you're on your own to divine the right kind of attire, the key is to understand the kind of party you're attending, Post said.
If the party is with close friends and family, go for it. "Bring out the kitschy candy cane earrings," Post said. Every family seems to have a fun person who loves to go bold for the holidays.
If it's a work-related invitation or anything formal, wearing anything shaped like a present, a candy cane or Santa Claus could be a huge manners mistake. "You need to either leave it at home or it needs to be so tasteful that it would be considered cute and sweet without being childish or kitschy," she said.
If you're going to your boyfriend's parents' house, where you will meet them for the first time, think twice about the impression you'd make by being decked out like Santa's helper from the mall.
Post recommends dressing in a single classically festive color, like red, green or cobalt blue.
"I'm a huge fan of black and gold or black and silver because that's always going to look pretty sharp," she said.
Or, as Courtney Weinblatt, the market director for Marie Claire magazine said, look to the resort season runway shows for what's holiday appropriate. "The runway trends can be the perfect guide," she said.
This year, fashions include sequined or lamé fabrics with beaded or fringe accents. Just don't wear that all at once.
"The key here," Weinblatt said, "is to remember that a little bit goes a long way."
Designers have taken a relaxed and pared-down approach to festive fabrics that have over-the-top reputations. (Fabrics like lamé have an indelible history: Think MC Hammer, "Dynasty" and Solid Gold Dancers.) Marc Jacobs used sequins on pants and paired them with a cashmere crewneck sweater for a playful, sporty look, she said, and Lanvin's T-shirt dresses featured gigantic, beaded flowers that looked soft and elegant because of tonal beading.
Even in the most formal of situations, clothing is more casual now than it has been in the past, she said. Festive dress is an opportunity to play up special elements, she said.
"(The holidays are) such a rare opportunity to dress up these days," Weinblatt said.
But what if your special element is a snowflake-and-jingle-bell necklace?
"Never say never," she said. "It's all in how you wear it. If you're going to have this crazy, Christmasy necklace, I would suggest wearing all black with that.
"But own it, if that's what you're feeling."
That's how she approaches outrageous holiday sweaters from the 1980s and '90s. Pair them with clean, understated makeup and hair, and have a great time.
"If you're in on the joke, then you can really run with it," Weinblatt said, "and if you're not, then God bless you and it's amazing that you still really love that sweater."
Prepare to be amazed, because many people do.
Anne Marie Blackman uses a sewing machine and a glue gun to modernize vintage sweaters for her business, MyUglyChristmasSweater.com. For Blackman, more is more.
"They're definitely festive," she said of her creations.
And, yes, they're also called "ugly."
But there's something about these vintage sweaters that makes you smile, she said. Perhaps younger generations wear them ironically, trying to out-ugly each other, Blackman said, and she offers them a degree of holiday flair that really shines.
She also remembers that in the 1980s, people truly loved these sweaters.
"Santa hats on geese is classic (early) '80s," Blackman said.
"Someone would have had home decorations that matched that sweater, and they weren't tacky at all, at that time. ...You can picture someone with embroidered geese in a deep brown frame."
While many other survivors of the 1980s might never bring winter geese back into their holiday wardrobe, modern notions of "festive" still include some fun, she said.
"In the holiday season people are feeling happy," she said. "A funny sweater adds to the festive flavor. If people dress in something that makes them stand out, they will have more fun."