If it's everyday wear... the "kilt".

If it's a Formal event, here's what the answer should be:

"The most important thing to remember when wearing Highland evening dress is that it represents a proud heritage and a proud people. Whilst one can take certain sartorial liberties, they should not be so outrageous as to offend more conventional guests!

There's that darned "Know Your Tartan" mentality again. Shall we add "Know When to Wear You Iffy Kilt" thought to the list?