In 1907, the city of Boulder passed an anti-saloon ordinance. This law said that saloons and bars could not do business in the city, and that people could not drink alcohol. In 1916, the entire state of Colorado started prohibition. In US History, the "Prohibition Era" was a time when all alcohol was illegal. People could not make, drink, buy, sell, or serve alcohol. Prohibition ended in 1933, when the Twenty-first Amendment to the United States Constitution was passed by Congress.