1929 was the year of the “modern” woman. She had liberties, among them stepping out to cocktail soirées and lounges between 6 and 8PM. Her “uniform” became a short sheath with matching hat, shoes, and gloves.
In the 1930s, Hollywood sirens like Greta Garbo were fashion catalysts, and were mixing French designer elements (think Chanel) with American sportiness. Thus, the dressy cocktail suit was born — to take one from afternoon tea to an intimate evening fête.
Then came the economic hardships of the 1930s, when American designers like Muriel King designed “day-into-evening” clothes — simple, streamlined silhouettes that emphasized the importance of accessories.
Hemlines began to rise. Parisian milliners produced elegant chapeaus with black silk net veils. And, in New York, designers were attaching rhinestone buttons to “vodka” gray or “billiard” green day suits to designate them cocktail ensembles.
By the late 1940s, Christian Dior was the first to name the early evening frock a “cocktail” dress.
The rest, as they say, is history.