When the invitation says “Semi Formal or Dressy Casual” first you may take your cues from the time of the event, for example, wear darker, more formal colors for an evening wedding, and opt for light colors and fabrics for a daytime event. If it’s daytime, then a sundress would be fine. If it’s evening, then wear cocktail-esque attire in darker, richer colors, and fabrics like silk or satin in a casual length and cut.
However, if you don’t own dress appropriate for the time of the event, you could also “trick” your look with makeup and accessories such as clutch, shoes, and jewelries. Light colored dress in the evening can be paired with little bold accessories along with makeup for evening wedding, while dark colored dress can go for daytime affair if matched with subtler and lighter accessories as well as makeup.
As for the formality of the dress, generally, the attire for this invitation suggests you to dress somewhere between formal and casual. It basically calls for dressed-up versions of casual looks. Meaning, you could think between a dress in casual fabrics like cotton or knits but with a formal cut, or formal fabrics like satin in a casual style.
While guys can wear a suit or a sport coat, and tie, the girls can opt for a cocktail dress or a dressy skirt and top. The length of your outfit (either dress or skirt) would be cocktail length—in other words, just above, or at the knee; or a little longer. A tea-length, or a longer, ankle-skimming hemline are welcome for this event. Even sometimes ladies are also allowed to wear dressy, tailored pants with chic blouse. Check out some ideas of semi-formal wedding guest dress from various different stores.