The 1920s decade covers the period from 1920-1929.
This decade on Earth is set between the First and Second World Wars. During that time, the Charleston is a popular dance. Some women are referred to as flappers. (TNG: "Emergence") The United States economy starts to deteriorate, an event which causes the Great Depression and has consequences even in an alternate timeline (see 1929). (TOS: "The City on the Edge of Forever"; ENT: "Storm Front, Part II") During the Prohibition era, organized crime rises to never-seen heights. The book Chicago Mobs of the Twenties chronicles this era, and serves as a guide for the civilization on Sigma Iotia II. (TOS: "A Piece of the Action")
Scientific breakthroughs on Earth include the launch of the first liquid propellant rocket. (TNG: "Loud As A Whisper" okudagram) Radio becomes a popular form of entertainment during this decade. (TOS: "The City on the Edge of Forever") In art, the film Metropolis is released and Irving Berlin writes the song "Blue Skies". (TOS: "The Cloud Minders"; Star Trek Nemesis) The song "Someone to Watch Over Me" is also released. (VOY: "Someone to Watch Over Me")