The Evolution of the Roman Calendar

The Roman calendar started out as a ten-month calendar of 304 days. Each year began on March 1st when the Vestal Virgins re-lit the sacred fire on Vesta's hearth, and fresh laurels were hung on public buildings.

The change to a 12-month calendar occurred sometime in the 6th Century BC. The year then became 355 days long and was configured as listed in the table below.

Month # of Days
Januarius 29
Februarius 28 (23 or 24)*
Intercalis (27)*
Martius 31
Aprilis 29
Maius 31
Junius 29
Quintilus 31
Sextilis 29
September 29
October 31
November 29
December 29

  • Quintilius was renamed Julius in 44 BC
  • Sextilis was renamed Augustus in 8 BC

*The Intercalary month or Intercalaris (aka Mercedonius) was used to synchronize the Roman calendar with the solar year.  Every other year, this intercalary month was supposed to be inserted between Februarius and Martius. In those years, Ferbruarius ended on the 23rd or the 24th, and the intecalary month of 27 days started. However, no intercalary month could be inserted unless official notice was first given by the College of Pontiffs. Unfortunately, they sometimes were motivated by corruption or by laxness and conveniently forgot to declare an Intercalaris.

In 153 BC, Januarius was made the first month of the year

By Julius Caesar's time, the calendar was really a mess because this system had resulted in an average of 366 1/4 days a year over each four year span. Over time, the disparity between the solar year and the Roman year was exacerbated to such a degree that by the end of 47 BC, the Roman calendar was almost two and a half months ahead of where it should have been.

So Caesar, in 46 BC, extended that year to 445 days to bring the calendar back in line. He then, from January 1, 45 BC onward, defined the year as having 365 days and introduced the leap year concept by adding an extra day between Februarius 23 and 24 every three years (this was corrected to four in 8 BC). The result we know as the Julian calendar.

How the Romans Referred to Their Years and Months

The Romans numbered the years from the year of the founding of Rome by Romulus. So 753 BC was year 1 AUC (ab urbe condita)

However, in conversation, they referred to specific years by the names of the Consuls who served for that year.

In the Roman calendar, the days of the month weren’t numbered sequentially. Instead, the months had three primary markers — the Kalendae, the Nonae and the Idus and all days were designated with respect to them.

All the days after the Kalends were numbered by counting down to the Nones (e.g., the day after the Kalends of Martius was referred to as the fifth day before the Nones of Martius). All days after the Ides were numbered by counting down toward the Kalends (e.g, the day after the Ides of March was referred to as the sixteenth day before the Kalends of Aprilis). All the days after the Nones were numbered by counting down to the Ides (e.g., two days before the Ides).

The Roman Week and How the Days Were Named

The Roman week was an eight day week which started with the Nundinae (Market Day). Until the acceptance of the seven-day week sometime after 19 BC, each of the week's eight days were referred to–in writing and in speech–as the Nundinae, the day before the Nundinae, two days before the Nundinae, and so forth. The other days in the Roman calendar week were designated as dies fasti, days on which legal action was permitted; dies nefasti, which meant that no legal action or public voting could take place on this day; and endotercisus, or intercisus, which were "in-between" days in which mornings and afternoons had different restrictions.

Festival days, or feriae occurred throughout the Roman year.The Romans enjoyed more holidays than the number of our holidays and weekends combined. (See Feasts and Holidays)

Other Notes

  • The Roman Nundinae was a day when traders and lawyers were at their busiest, and there was no ban at all on work
  • The first public evidence of the seven-day week in Rome occurred in a Sabine calendar of between 19 BC and 14 AD. By 79 AD, there was widespread familiarity with the seven-day week and the corresponding names of the days
  • The interval between the first and third of three successive Nundinae was called the trinundinum