Sextilis ("sixth") or mensis Sextilis was the Latin name for what was originally the sixth month in the Roman calendar, when March (Martius, "Mars' month") was the first of ten months in the year. After the calendar reform that produced a twelve-month year, Sextilis became the eighth month, but retained its name. It was renamed Augustus (August) in 8 BC in honor of the first Roman emperor, Augustus. Sextilis followed Quinctilis, which was renamed Julius (July) after Julius Caesar, and preceded September (from septem, "seven"), which was originally the seventh month.

The month Augustus

Julius (July) was renamed from Quintilis ("fifth" month) in honor of Julius Caesar, who had adopted his grand-nephew Octavian, the future Augustus, and made him his heir. It has sometimes been thought that the month has 31 days because Augustus wanted as many days in his month as in his predecessor's, but Sextilis in fact had 31 days since the reform during Caesar's dictatorship that created the Julian calendar.