|Make the most of your Fred and/or Ginger moment|
When?Extremely rarely, for the average Jo(e), unless you happen to entertain heads of state, or attend very formal weddings, balls, banquets or galas post-7pm on a regular basis. Usually specified on the invite as "White Tie" or "Full Evening Wear". So, if you're off to the ballet/opera, do yourself a favour and leave the top hat at home - sad as it may be, you'll probably feel a little silly if the person sitting next to you is in top-to-toe denim.
For guys, you guessed it, a white bow tie is crucial, teamed with matching waistcoat, shirt and wing collar, plus a black, single-breasted tailcoat and, traditionally, double-braided trousers. Shoes should be black and, traditionally, lace-up and shiny-as-you-like.
For girls, full length and fabulous is the order of the day. Shorter dresses or, quelle horreur, trousers, are big no-nos. Tiaras are traditionally restricted to married women, but other forms of statement jewellery are equal-opportunity-friendly.
When you need to go formal, but not "seated next to the Obamas" formal - sometimes specified as "dinner jackets", "dress for dinner" or "cravate noir".
For guys, this is tuxedo territory - a black, single-breasted suit blazer with silk peaked lapels and matching single-braided trousers teamed with white evening shirt, black bow tie, and those shiny-shiny black shoes again. Waistcoats and cummerbunds are both optional - but please remember this is an either/or situation - don't double up!
For girls, fancy dresses of any length go (provided you exercise a certain degree of decorum), and wide-legged evening trousers such as silky palazzo pants are also an option. Costume jewellery is encouraged, but I'm afraid you'll have to leave the tiara at home this time, whatever your marital status.
For the least formal of formal occasions (such as, er... the Oscars and the Met Gala, according to these maverick male celebs) - it's time to get a bit more creative.
Guys should wear dark-coloured suits and ties, but avoid black. This is your time to shine in patterned ties and, if you're feeling brave, coloured socks - you still need to polish those shoes, though.
For girls, cocktail dresses traditionally hit just above or below knee level - the LBD is always a safe bet, but brocade cigarette pants are a bolder (and Sam Muses-endorsed) alternative.
Here endeth the lesson. Go forth in formal wear.