When Everybody Ate at Schrafft's
Quite an interesting Emily Post 1922 citation in general,
http://www.bartleby.com/95/33.html
but this specifically - (italics mine)
WHAT TO WEAR IN A RESTAURANT
Restaurant dress depends upon the restaurant and the city. Because women in New York wear low-necked dresses and no hats, does not mean that those who live in New Town should do the same, if it is not New Town's custom. But you must never wear an evening dress and a hat!
And never wear a day dress without one. If in the city where you live, people wear day clothes in the evening, you can only very slightly differ from them.
67
It is never good form to be elaborately dressed in a public place, except in a box at the opera or at a charity ball.
Of interest for men, The Evil Swede's Guide to Hat Etiquette -
http://www.bcvc.net/hats/index.htm
It's all tempera et mores.