Formal, semi-formal, casual and business casual are all different styles of dress for certain events. Semi-formal attire requires you to dress up in a nicer outfit without having to wear a full tuxedo or floor length dress. It's a bit more casual and allows you more freedom with the choice of your outfit.
During the evening, a semi-formal outfit for men include a dress shirt with a dark business suit. Men should also wear a tie. During the day, men can wear a linen suit with khaki pants and a tie. For a dinner, men can wear a black suit, with a light dress shirt and tie to match. For a wedding, men should wear a dark suit with a light dress shirt and a matching tie, much like men would wear to a semi-formal dinner. If the wedding is a bit more casual, men can wear a lighter suit with pressed khaki pants or dark pants. Men don't have to wear a tie if the event is more casual.
Women should wear a short cocktail dress that hits at or above the knees. Other acceptable items include a little black dress, separates that are dressy (no jeans) or a skirt that's long and dressy with a nice top. For a woman who doesn't wear dresses, a dressy pant suit can be worn to a semi-formal event. Fabrics that can be considered are chiffon, satin, taffeta and gossamer types of fabric. For a dinner, women should wear a little black dress. For a wedding, women could wear a cocktail dress in lighter colors.
Men should wear a type of dark leather shoe and nice socks that are dark in color. Women should wear color-coordinated dress shoes with their outfit. These should be heels unless the rest of the outfit is dressy enough for flats. For example, flat dressy sandals or ballet flats could be worn by women with their outfits.
Women should wear stud earrings and a bracelet or necklace with their outfit. Colors of the jewelry should match or play off the rest of the outfit. Men should wear metal cuff links and a dressy watch. Women should carry a small clutch purse that matches their outfit. Men should wear a dark leather belt with their outfit as well. If the semi-formal event is during cold months, women could add a silk scarf or shawl.