In the past, semi-formal dress codes were specific and rigid. The first appearance of the "dinner jacket," the beginning of semi-formal attire, dates back to the late 19th century. Prince Edward VII of Wales instructed his tailor to make a jacket without tails for a friend he invited to dinner. This jacket became known as the tuxedo. Today, tuxedos would be considered too formal to wear to a semi-formal event. The range of what is acceptable, for both men and women, is much wider.

For Men

Wear a complete suit (vest optional). A sport jacket, with a shirt, no tie and dress slacks is not appropriate. Wear a dark-colored suit. Lighter colors in light fabrics such as linen or seersucker are not appropriate. A suit made of wool gabardine, cashmere or wool will do. A button-down shirt and a complementary tie completes the look. If you're unsure about what kind of patterned tie to wear, simply choose a solid-colored silk tie. Wear polished dress shoes--black is always safe. If you prefer brown shoes, make sure they're dark brown and pair them with a navy rather than a black suit. Accessorize with a good watch or cuff-links. Simple is better.

For Women

The line between traditional semi-formal dress and cocktail is blurry. The difference is in hem length. Typically, a semi-formal dress should fall between 1 inch above the knee and a few inches above the ankle; any longer and it's a gown. A cocktail dress rarely falls below the knee. A cocktail dress may be appropriate for semi-formal wear at a day wedding. Women have many choices in color, fabric and accessories. If you stay within a few guidelines, you're only limited by your sense of personal style. If in doubt, always choose solid colors in darker shades. Fabric choice is up to you, but woven fabrics (such as silk or brocade) are preferred over knit fabrics such as cotton, or stretch polyester. Wear heels--small kitten heels or higher. Flat sandals are not recommended, including "dressy" sandals. You have your choice of straps, but be wary of anything that shows a lot of decolletage or undergarments that peep through. Jewelry is also a personal choice. If the dress has color or pattern, keep the jewelry simple and minimal. If the dress is dark and simple, jewelry can be colorful, larger and more plentiful. If you object to wearing a dress, a tailored women's pantsuit is also acceptable. Choose a dark color, wear a small heel and try a silk top beneath.

Source:

"Dressing the Man: Mastering the art of Permanent Fashion"; Alan Flusser; New York: HarperCollins Publishers, Inc., (2002). p. 240.

Lighter colors in light fabrics, such as linen or seersucker are not appropriate.

Woven fabrics (such as silk or brocade) are preferred

Resource:

What is Semi-Formal Attire?