Only 10 Months at First
The Romans borrowed parts of their earliest known calendar from the Greeks. According to tradition, Romulus, the legendary first king of Rome, oversaw an overhaul of the Roman calendar system around 738 BCE. The resulting calendar, had only 10 months, with March (Martius) being the first month of the year. The 10 months were named Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December – with the last six names were taken from the words for five, six, seven, eight, nine, and ten. The winter season was not assigned to any month, so the year only lasted 304 days, with 61 days unaccounted for in the winter.
Republican Calendar Adds January and February
Following another calendar reform, which later Roman writers attributed to Romulus’ successor, Numa Pompilius, the Republican calendar was instituted. To account for the days of winter between the years, two additional months were introduced: Ianuarius (January) and Februarius (February).
This meant that some of the month names no longer agreed with their position in the calendar. For example, September means “the 7th month,” but it was now the 9th month of the year—an inconsistency that was preserved and is still part of the Gregorian calendar we use today.
A common year was now divided into 12 months of different lengths: 4 “full” months with 31 days, 7 “hollow” months with 29 days, and 1 month with 28 days.
The Leap Month, Mercedonius
The Republican calendar year lasted for 355 days, which is about 10 days shorter than a tropical year, the time it takes Earth to revolve around the Sun. To keep the calendar in sync with the seasons, a leap month called Mercedonius or Intercalaris was added in some years—normally every two to three years.
By custom, the insertion of the leap month was initiated by the pontifex maximus, the high priest of the College of Pontiffs in ancient Rome. However, this system was vulnerable to abuse. Since the Roman calendar year defined the term of office of elected officials, a pontifex maximus was able to control the length of his term simply by adding a leap month.
When Julius Caesar became pontifex maximus, he ordered a calendar reform which eliminated leap months and resulted in the implementation of the Julian calendar in 45 BCE, the direct predecessor of today’s Gregorian calendar.
Calends, Nones, and Ides
The Roman calendar highlighted a number of days in each month:
Calends (Kalendae) were the first days of each month. The name is derived from the Greek word καλειν, to announce, which may initially have been used in the ancient lunar calendar to “announce” the day of the New Moon (or the first sliver of the Waxing Crescent Moon).
Ides (Idus) occurred one day before the middle of each month. Depending on the month’s length, it fell on the 13th or 15th day. In the lunar calendar, the Ides marked the day of the Full Moon.
Nones (Nonae) fell on the 7th day of 31-day months and on the 5th day of 29-day months, marking the day of the First Quarter Moon.
These markers were used to number the days in each month, counting backward from the upcoming Calends, Ides, or Nones. The count always included the day of the marker. For example, the 11th day of Martius would be known as “Five Ides” to the Romans because it is the fifth day before the Ides of Martius, which fell on the 15th day.
Days of the Week
At the time of their early kings, Roman months were of a length identical to the lunar cycle. Each month was divided into sections that ended on the day of one of the first three phases of the moon: new, first quarter or full. All days were referred to in terms of one of these three moon phase names, Kalends, Nones or Ides.
The Romans did not have weekdays in the same sense as our Monday, Tuesday, etc., however, they did have a defined markers within each month. Originally, the month and the markers were based on the moon.
At that time a pontifex (priest) was assigned to observe the sky. When he first sighted a thin lunar crescent he called out that there was a new moon and declared the next month had started. For centuries afterward, Romans referred to the first day of each month as Kalendae or Kalends from the Latin word calare (to announce solemnly, to call out). The word calendar was derived from this custom.