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"Then turning his attention to the reorganisation of the state, he reformed the calendar, which the negligence of the pontiffs had long since so disordered, through their privilege of adding months or days at pleasure, that the harvest festivals did not come in summer nor those of the vintage in the autumn; and he adjusted the year to the sun's course by making it consist of three hundred and sixty-five days, abolishing the intercalary month, and adding one day every fourth year. Furthermore, that the correct reckoning of seasons might begin with the next Kalends of January, he inserted two other months between those of November and December; hence the year in which these arrangements were made was one of fifteen months, including the intercalary month, which belonged to that year according to the former custom."
Suetonius, Life of Julius Caesar (XL.1-2)
The Republican calendar had four months of thirty-one days (March, July, October, and May); the other months were short, with twenty-nine days, except for February, which had twenty eight--all of which adds up to 355 days. To adjust that number to 365, Julius Caesar added one or two extra days at the end of the seven short months, bringing their totals to thirty or thirty-one days. The position of the Nones and the Ides, therefore, were not affected, as were the festivals and anniversaries that counted down to these two fixed points on the calendar. The second half of the month, however, after the Ides, when the Romans began to count down to the Kalends of the next month, was affected by the addition of these new days. A celebration that fell between the Ides and the following Kalends in a month whose length had changed either could remain on the same date or the same day, but not both. In other words, an anniversary that had fallen so many days before the Kalends in the old calendar could retain its position on the Julian calendar, even if the Kalends, itself, had shifted one or two days farther away. But now, even if the date were the same, the day was not. Determined by the following Kalends, it had moved farther away from the Ides. Given the importance of when the festivals traditionally had been celebrated, they retained their same "day," the number of days after the Ides. But their notational "date," their position relative to the following Kalends, changed.
For example, the birthday of Octavian (the future Augustus) is September 23. In the Republican calendar, the month had twenty-nine days and the date was a.d.VIII.Kal.Oct. The Julian reform, however, added one more day to September to make it a thirty-day month. There now was one more day before reaching the Kalends of October, which displaced "the eighth day before the Kalends of October" by a day. Octavian's birthday is on the same number of days after the Ides, but it is no longer eight days before the Kalends of October, but nine (a.d.IX.Kal.Oct). When Suetonius (Life, V) recorded that the future Augustus was born on "the ninth day before the Kalends of October in the consulship of Marcus Tullius Cicero and Gaius Antonius," the question now is whether he is giving a Republican date (corresponding to September 22 in a twenty-nine day September) or a Julian one (corresponding to September 23 in a thirty-day month). That it is the latter can be seen in that the date corresponds to the festival of Apollo, whom Octavian revered. One can assume that, having been born in 63 BC (under the Republican calendar), he would, after the Julian reform on January 1, 45 BC, wanted to celebrate his eighteenth birthday at a time that coincided with the feast day of Apollo. The day stayed the same (the same number of days after the Ides), although the date, itself, changed (nine, not eight, days before the Kalends).
Reference: Caesar's Calendar (2007) by Denis Feeney.
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