A semi-formal dress code falls somewhere between casual and formal. For men, a dark suit would be appropriate, while a knee-length cocktail dress is suitable for women.
As the name implies, a semi-formal dress code is not completely formal, but it is more formal than casual or business casual. If an invitation indicates a formal or black tie dress code, men would be expected to wear a tuxedo and women would typically wear full-length gowns. Semi-formal is just one step below formal; therefore, tuxes and ball gowns would be excessive.
If the event takes place in the evening, men should choose a dark suit with a coordinating tie over a white or solid-colored button down shirt. If it is a daytime event, a shirt or tie with a subtle pattern is acceptable. For women, a cocktail dress with dressy shoes and accessories are appropriate. For a daytime or business event, a tailored suit or dress is acceptable.