Roman Calendar

Of all the calendars around the ancient world in Europe, the Roman calendar survived to this time. However, the calendar underwent several evolutionary changes.

According to Roman legend, the calendar used by the Romans began at the time of the establishment of the Roman monarchy. Romulus, the founder of Rome, created the Roman calendar of 304 days with ten months, with the New Year starting with month of March. The months after June were Quintilis (fifth month) and Sextilis (sixth month). December was the tenth month of a year.

Numa Pompilius, Romulus' successor, added two more months to the end of the calendar: January and February. So Numa's calendar had 355 days. The first Etruscan king of Rome, Tarquinius Priscus, wanted to move the New Year to January, but this reform was abolished when the Republic was established; the New Year was reverted back to March. The Roman calendar was based on the lunar calendar system of 355 days. An intercalary month (27 or 28 days) was added to the end of February, to prevent the calendar from being too far out of phase with the seasons and the solstices.

This arrangement of the calendar remained unchanged through much of the period of the Roman Republic. The first of March was the day when two consuls were elected. The consuls were official magistrates that had the power of a Roman king and could command an army, but they only held office for a term of one year.

It wasn't until the first century BC that the Senate decided to move the New Year and the election of the consuls to the 1st of January. This was a time before Julius Caesar (died in 44 BC) came into power.

It was Julius Caesar who developed the new calendar system, which would be called the Julian Calendar. Caesar worked with an Alexandrian astronomer named Sosigenes to calculate the solar calendar year of 365 1/4 days (or 365 days and 6 hours). The names of the months were kept the same. Getting rid of the intercalary month from the lunar calendar, the calendar would still be slightly out of phase with the seasons. To correct this – at the interval of every four years, another day would be added to the end of February, so that it would have 29 days instead of 28. This year would be known as the Leap Year.

During or after the reign of the first emperor, Augustus Caesar, the great-nephew of Julius Caesar, the month of Quintilis was changed to July after Julius Caesar, while the month of Sextilis was changed to August, after Augustus.

Early Roman Calendar (Romulus)	Early Roman Calendar (Numa)	Late Roman Republican Calendar (pre-Julian)	Julian Calendar
March (named after Mars)	March	January	January
April	April	February	February
May (named after Maia)	May	March	March
June (named after Juno)	June	April	April
Quintilis (5th month)	Quintilis	May	May
Sextilis (6th month)	Sextilis	June	June
September (7th month)	September	Quintilis	July (named after Julius Caesar)
October (8th month)	October	Sextilis	August (named after Augustus)
November (9th month)	November	September	September
December (10th month)	December	October	October
	January (named after Janus)	November	November
	February (named after Februus)	December	December

However, the Julian Calendar was not perfect, because the true solar year had 365.242199 days, not 365.25 days. So calendar was still out of phase with the seasons (the Equinoxes and Solstices), because the Julian Calendar had a leap year on the year of a new century or centennial year. So the Julian calendar was out by 11 minutes and 14 seconds to the true calendar year.

A little side note. The solar calendar is 365 days, 5 hours, 48 minutes and 46 seconds (all of which is equal to 365.242199 days).

It wasn't until 1582 that Pope Gregory XIII made an amendment to the Julian calendar to correct the error. Instead of having a leap year on each new century, the leap year would happen at every four centennial years. So the centennial year would have no leap year unless it was divisible by 400. So the year 1600 was a leap year, while there were no leap years in the year 1700, 1800 and 1900. The year 2000 was a leap year (but I think I had slept through it).

The new amendment caused the new calendar to be known as the Gregorian Calendar. This is the calendar system that we are still using today. Not everyone was happy with the new reform at the time however, mostly because the Gregorian calendar made it more complicated to calculate Easter festivals.

Worse than that, since the old calendar had a leap year in every centennial year, the Julian calendar was out of phase by 11 days with the equinox by the time of Pope Gregory XIII. To fix this, eleven days was removed from the calendar when the Gregorian Calendar was adopted. But this caused minor peasants' revolts. The peasants bitterly complained that the authorities stolen their 11 days, and they wanted those missing days back.

The Gregorian calendar was not universally accepted by some of the countries of Western Europe. The traditionally Catholic countries such as Italy, Spain and Catholic Germany mostly adopted the new calendar. The Protestant German states didn't adopt the calendar until 1699, while England (the United Kingdom) didn't accept the new calendar until 1752. Greece made the change to the Gregorian calendar as late as 1923.

Below, I have included a table on the days that were dedicated to the Roman gods, with the modern equivalents.

Roman Days	Identification with God/Goddess	Modern Equivalent
dies solis	Sol	Sunday
dies lunae	Luna	Monday
dies Martis	Mars	Tuesday
dies Mercurii	Mercury	Wednesday
dies Iovis	Jupiter	Thursday
dies Veneris	Venus	Friday
dies Saturni	Saturn	Saturday