I’ve posted many a time about what it means when the invitation says “black tie.” You can read more about that here.
What’s the short version? Don’t show up in your cotton daydress. Put on the jewelry that you feel is a little too fancy for work or the playground. DRESS UP. You should feel comfortable in a cocktail dress.
One other thing to keep in mind. Solid color dresses may wind you up in the bridesmaid category. So be weary. Also, NEVER wear a white dress to a wedding. The End. 🙂