A step down from formal, semi-formal attire is neither black-tie apparel nor cocktail garb. Where black tie constitutes tuxedoes and formal gowns, cocktail equates with a loose, somewhat sexy atmosphere. Semi-formal dress falls into a vague area in between. When confronted with the invitation to attend a semi-formal event keep in mind the following.

Events such as weddings, award ceremonies, bar and bat mitzvahs, and attending the theatre are all times when semi-formal is usually the expected dress code. This applies, however, to daytime events. If the above examples take place in the evening then a more formal look would be required.

Not to be confused with cocktail attire, semiformal is indeed much more formal. Day semiformal for a man is a look which traditionally consists of a jacket, trousers, vest and tie. Unless you are a member of the wedding party, you are not required to be dressed up in a tuxedo. Light colored suits and a tie allows you to fit in with style. Clothing in wool, polyester and gabardine and great options. And remember that any evening event still calls for a dark suit at the very least. Finally today’s fashion world has begun to dictate that ties are not absolutely necessary to complete the look at a semiformal event.
Linen or seersucker suits are not formal enough for semi-formal attire. Dress shoes like oxfords are worn.
For women, dresses at the knee and below or a stylish pant suit are fantastic options. Fabrics using taffeta, chiffon, velvet, satin, sateen, gossamer, and velour are great to women. A dress suit is also a stylish look. Opt for one in silk or brocade and you’ll look stunning in your semi-casual attire. Except for cashmere, a wool business suit is not considered semi-formal attire since it is not dressy enough.
Hem lengths in semi-formal attire dresses vary. Dresses can be cocktail dresses, no more than an inch above the knee, or they can extend to the ankle. Dress suits, for example, a silk suit, usually has a hem length slightly above, at or below the knee. Pants suits, again in silk, polyester, or cashmere, tend to be worn at the ankle. One can also wear dressy tops and tuxedo pants made out of appropriate fabrics and still fit into the semi-formal attire group.
Generally semi-formal attire also means dress heels for women, usually in colors coordinating with one's outfit. However, a dressy flat or strappy sandal with the appropriate outfit is also considered semi-formal attire. Low, comfy flipflops and lofers are not dressy enough to be called semi-formal attire.