As we approach that time in the calendar when we celebrate getting through another year and look ahead to the next one, thoughts inevitably turn to what one should wear. Why is the well-meaning sartorial advice on party invitations always so unhelpful? “Black tie” or the dreaded “smart casual” are probably some use to chaps, at least letting them know whether a tuxedo, a lounge suit or no suit at all is required. For women such phrases just add to the sense of the impending minefield. Neither addresses the most basic question—whether to go for long or short? The reason that most suggestions on attire do not state any rules is really quite simple—there are none. It is possible to look far more dressed up in an all-singing, all-dancing, spangly short frock than in a plain black long dress. Or is it? There is something inherently more frivolous about a highly embellished short dress, so let the gravitas of the occasion be your guide. And if anyone is sloppy enough to send out a “smart casual” injunction, as the hosts they are the ones who should feel uncomfortable if guests get it wrong.

No one wants to find themselves the 21st- century equivalent of an HM Bateman cartoon, as The Woman Who Went to a Beer’n’Burger Night in a Ball Gown, but the occasion and the venue provide useful clues. If it is a ball, then a long gown— the grander the better—is the only appropriate wear, unless you happen to have great legs and can lay your hands on a beaded haute couture dress by Dior or Chanel, some of which are really very short indeed this winter but command all the respect that their breeding implies. Black tie is far more nebulous, but the smarter the venue, the more a long dress will feel at home. I have one chic friend whose rule is always to wear long at a black-tie event, not out of etiquette but because it is, she says, a rare chance to dress up. However, some long frocks are dressier than others. There used to be something called a “dinner dress”, long but simple, and there is a move afoot in fashion’s current modernist mood to reinvent it, under any name but that. Simple shapes, where the secret is in the cut and fabric, offer versatility. Add fabulous jewels and skinny stilettoes, or a pair of flat sandals and diamond studs, and you can go anywhere.

Short evening dresses (confusingly called “cocktail frocks” and perfect for drinks ) have their own codes, for the journey from knee to thigh is fraught with pitfalls. A simple LBD hits the spot for a formal dinner or a full-on party in a sleek, modern venue. Now that flat shoes are back, the rule is the shorter the skirt, the lower the heel. Only wear a micro-mini and those towering heels jocularly referred to as “Saturday night Cheryls” if you wish to create a sense of irony and are attending the sort of party where you might be required to record your posterior for posterity on the photocopier under the mistletoe. But be aware that your intentions may be misconstrued, unless your dress is an aforementioned Chanel or Dior version, in which case your act will be pure class.

Avril Groom is a writer and editor specialising in luxury and fashion