Nightlife—the nightlife that Americans know now, with dark restaurants and dance floors—did not exist until the 1920s. It's not that people didn't go out at night. But the way they went out, to saloons where women weren't welcome, to vaudeville shows, to balls and organized dances, wasn't anything like the way many go out at night today.
Before the 1920s, in big cities and especially in New York City, it was possible (for men, at least) to go out at night for food or drink or entertainment. In New York, in the first couple decades of the 20th century, as the population boomed, all these activities started to concentrate at a new kind of establishment: nightclubs. In Nightclub City, the historian Burton Peretti writes that by 1920:
[T]he term "nightlife" especially referred to specific kinds of places for dining, drink, socializing, and intimate entertainment…Some cabaret owners bought up New York state charters that had been issued to now-defunct voluntary associations, to evade the city's closing-time laws. The concept of the "nightclub" thus was born.
And then, almost as soon as this idea had started to take hold, Prohibition tried to shut it down.
For a hot second, it worked—the new nightlife establishments withered, and many closed. But within a few years, nightclubs were back and thriving in the city in the form of underground speakeasies and trendy jazz clubs. This was when something that we would recognize as nightlife really began. For the first time, women went out to drink too and occupied the same small, dark spaces as men. Couples started dating, instead of courting. Wealthy white people went to Harlem to listen to music and dance, and while sometimes that went as badly as one might expect, this counted as progress. Here's how writer Anais Nin experienced it:
Harlem. The Savoy. Music which makes the floor tremble, a vast place, with creamy drinks, dusky lights, and genuine gaiety…The rhythm unleashes everyone as you step on the floor. Rank said he could not dance. 'A new world, a new world,' he murmured, astonished and bewildered.
It wasn't just at clubs that the patterns of drinking and socializing were being set either. Reporting in Last Call, the writer Daniel Okrent found that house parties was becoming popular, too:
In 1923 after two years in Paris, critic Malcolm Cowley marveled at the invention of "the ‘party,’ conceived as a gathering of men and women to drink gin cocktails, flirt, dance to the phonograph or radio, and gossip about their absent friends."
If this sounds cool—well, it was in the 1920s that "cool" settled into its role as a word denoting cultural desirability. These years established the 20th century's idea of what cool could be. Bars have been been working off that notion ever since.