A quick history of the “cotillion” and the dresses normally involved with the function.

Taken from http://en.wikipedia.org/wiki/Cotillion …
The dance known as Cotillion came from France in the 1700s and was originally made up of four couples in a square formation. The Cotillion was one of many contredanses or “country dances” where the participants (especially those from royalty) gathered to introduce themselves and to flirt with other dancers through the exchange of partners within the formation network of the dance. By the 1800s, the Cotillion evolved into more couples with many complex dance figures and was the forerunner of the Quadrille. American square dance is a form of contredanse that also descended from the Cotillion. The direct translation from French to English of the word “Cotillion” is the word “petticoat’ but the more appropriate translation is the word “ballgown.”

A Debutante Ball is a formal presentation of young ladies, debutantes to polite society. It is in use in the United States more so in the South, and some other countries. Debutantes are usually recommended by a distinguished committee or sponsored by an established member of elite society. Modern debutante balls are often charity events; the parents of the debutante donate a certain amount of money to the designated cause, and the invited guests pay for their tickets. These balls may be elaborate formal affairs and involve not only “debs” but junior debutantes, escorts and ushers, flower girls and pages as well.

In the United States, Cotillion has become training that children and young adults attend to learn manners and proper social behavior in the context of formal dance. The participants dress up in formal attire and some cotillions emphasize the use of white gloves. Modern day Cotillion takes the form of classes and parties. These programs use dance as an interactive tool that breaks down inhibitions and teaches important social skills. Some of the dances currently taught include waltz, fox trot, tango, swing, cha cha and many other ballroom dances. The parties are typically “graduation” parties usually held after a series of classes. At the party, the students sit together in tables in the front while the family members sit in tables in the back. A master of ceremony guides the evening and, if funds are sufficient, dinner and an orchestra may be included. Usually after each dance, a couple may be recognized with a modest prize for best executing that dance.

On to the CB’s Limited interpretation: The dresses used for fall/spring cotillion should be semi formal/formal. Light formal and a step above should be used, but nothing overly formal. It should be a reasonably “fun” event, with lightweight gowns dominating the dance floor. Don’t overdo it with something that won’t let you move freely during dancing…you need something that flows and moves with the body. The fall/winter cotillion consists of darker colors like that of red/black/navy, etc., while spring cotillion has lighter colors such as turquoise, pink, green, etc.