Julian Calendar

   Early Romans sometimes inserted a 22 or 23-day month into their calendar after the 23rd day of Februarius. Called either Mercedinus or Intercalaris, its purpose was to keep holidays in their historical place within the solar year. However, for political or other reasons, by the time of Julius Caesar that month was being observed only now and then—not every second year as was originally intended. Because of this, by 46 B.C. holidays no longer occurred during the same season as they had near the time of Rome’s founding.

In order to return festivals to their original position, Caesar extended that calendar year to a total of between 443 and 445 days, depending upon which historian one believes. 1   Many modern historians have accepted the figure of 445 days for 46 B.C., that so-called “year of confusion.” Caesar, taking advice from an astronomer named Sosigenes, eliminated the month of Intercalaris and assigned all months other than February either 30 or 31 days. This extended future calendar year lengths to 365 days in common years and (by adding an extra day every fourth year) 366 in leap years. These changes resulted in the new Julian Calendar which converted their old nominally lunar calendar to one that was truly solar.
It’s sometimes said that Caesar’s new calendar was copied from the Alexandrian, but the only Alexandrian feature copied was its average year length of 365.25 days. 2
Romans began using their new Julian Calendar in 45 B.C. with what some sources claim was a leap year. But this belief that 45 B.C. was not a common year is probably untrue. A leap year should not have been needed because it followed the year of confusion, whose length had been extended the number of days required to put future calendars into a desired relationship with religious holidays.
Caesar had intended that during every succeeding fourth year an extra calendar day beyond the 365 in common years would be observed. It is believed that for about four decades after Caesars’ death, priests in charge of the calendar erroneously inserted an extra day every three years instead of every four. If true, this would have caused calendar dates to slowly drift away from their original seasons.

Soon after the first Caesar’s assassination, his nephew and adopted son Octavian, shown to the right, became the emperor that we refer to as “Augustus.” According to the fifth-century Roman writer Macrobius, the 3-year leap year mixup happened but was corrected by Augustus who eliminated intercalation for a period of 12 years.

Some claim the month of Sextilis was renamed for Augustus by Gaius Asinius Gallus and Gaius Marcius Censorinus who served as Ordinary Consuls of the Roman Republic in 8 B.C.E. Possibly they also extended that month’s length from what had been 30 to 31 days at the same time by reducing the length of another month by one day.
The revised Julian calendar served its users well for a long time. However, adding an extra day every four years resulted in extending the calendar over eleven minutes beyond solar year length every calendar year. This was not perceived as a problem for hundreds of years, but by the middle of the sixteenth century of this era, these added minutes had accumulated to point that Christian festivals were not being observed during their original time frames.
This problem had been known and discussed for several centuries, but the Julian Calendar was not revised further until the time of Pope Gregory, near the end of this era’s sixteenth century. You can read about changes made then that resulted in our present Gregorian Calendar.
Don’t confuse the Julian Calendar with Julian Numbers. They are two completely different things. See Calendrical Terms for a definition of the latter.

FOOTNOTES:
(1) Kepler, Johan. Letter to Herwart. A.D. 1597
(2) The Alexandrian Calendar had twelve months, each 30 days in length, in Julius Caesar’s time. In order to keep that calendar in phase with the solar year, 5 “epagomenal” days were observed after the twelfth month in three out of each four years. Every fourth year, 6 epagomenal days were added rather than 5.