What is a proper evening gown length?

The length depends on the formality of the occasion. For semi-formal events (when men wear suits), a cocktail dress is fine. For formal "black-tie" events, tea-length (mid-calf) to Ballet-length (ankle-length) gowns are appropriate. For most formal "white-tie" events, gowns should be ballet-length or full-length (floor-length). (Full-length gowns are sometimes called "Opera-length".) Debutantes should wear full-length gowns for their debuts. By the way, evening dress should usually be worn only after 6:00 p.m.