One of the reasons I enjoyed watching "Sex and the City" or "Gossip Girl" so much was because of all those fancy-but-still-casual parties they had there. So many opportunities to wear beautiful dresses that are too elegant for work/everyday but too modest for big events like a wedding.
Little Wikipedia research: cocktail parties started around 1917. Mrs. Walsh invited 50 guests to her house on a Sunday at high noon for a one-hour affair. "The party scored an instant hit," the newspaper declared and stated that, within weeks, cocktail parties had become "a St. Louis institution"
Is going out with friends our modern replacement for cocktail parties? Hmmm, maybe it's time to organize a little (cocktail) party :)