Complex sentence: By 1930, when prohibition was marking its tenth year in the United States, Glen Island Casino was acquiring the reputation as being a speakeasy, yet at the same time the Casino had also begun to book up-and coming musicians for weekend dances.
Split into simpler sentences: By 1930, when Prohibition was marking its 10th year in the United States, Glen Island Casino was acquiring a reputation as a speakeasy. At the same time, the casino had also begun to book up-and-coming musicians for weekend dances.