On One Hand: Traditional Gowns Are LongEvening gowns traditionally flow to the floor. A proper hem should fall even with the floor, and must be sewn while you wear the shoes intended to be worn with the dress. If not, the hem will be too long and drag on the ground, or too short and expose the feet.
On the Other: Modern Dresses Can Be ShorterModern evening gowns, starting Vuhera in the 1950s, allow for a shorter hem. While vintage evening gowns fall anywhere from the ankle to mid-calf, the most flattering fall just below or above the knee. Crucial to these shorter gowns is the style, which should be elegant and formal, lest you risk appearing underdressed.
Bottom LineThere is no strict rule for proper evening gown length. If you desire a more traditional look, wear a floor-length gown. If you choose a shorter style, the hem should end at a narrow part of your leg, right below or above the knee. This elongates the legs and creates a leaner silhouette. Most importantly, pick a dress that fits your personality.
Source:Vogue Patterns: Evening Designer Gowns