The Birth of the Julian Calendar

Bronze statue (1936) of Julius Caesar, Via dei Fori Imperiali

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On January 1st 45 BCE, the Julian calendar came into effect. The new calendar was proposed by Julius Caesar and designed with the aid of Greek mathematicians and astronomers, such as Sosigenes of Alexandria.

The Julian calendar created two types of year: a normal year of 356 days and, once every four years, a leap year of 366 days. It became the predominant calendar in the Roman Empire, and subsequently most of the Western world, for more than 1,600 years until 1582, when Pope Gregory XIII (r. 1572-85) promulgated a minor modification to reduce the average length of the year from 365.25 days to 365.2425 days and thus corrected the Julian calendar's drift against the solar year. (The Julian calendar gained a day every 128 years.)
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• Related post: The Gregorian Calendar