The remaining months they named, from the order in which they came, the fifth, sixth, seventh, eighth, ninth, tenth: Quintilis, Sextilis, September, … - Plutarch

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The remaining months they named, from the order in which they came, the fifth, sixth, seventh, eighth, ninth, tenth: Quintilis, Sextilis, September, October, November, December. Then Quintilis was called Julius after Julius Caesar, who conquered Pompeius; and Sextilis was called Augustus, after the second of the Roman Emperors. The next two months Domitian altered to his own titles, but not for any long time, as after his death they resumed their old names of September and October. The last two alone have preserved their original names without change.