The Roman Year
During the late Roman Republic, the length of the Roman year and its alignment with the seasons were, as a Roman muli in the legions would say, perfututi… totally screwed up.
In theory, the Roman calendar in use at that time was developed during the reign of the legendary second king of Rome, Numa Pompilius, around 713 BCE. The Roman year had 355 days divided unevenly into twelve months, Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, December.
In order to keep the calendar aligned with the seasons, the Romans inserted an intercalary month of twenty-two or twenty-three days in alternate years. This mensis intercalaris was typically placed within the month of Februarius (Macrobius, Saturnalia, 1.13.12-5).
That’s the theory, anyway.
In fact, by the time Insubrecus arrives in long-haired Gaul, the Roman calendar had lost coordination with the seasons.
The system of aligning the year with the seasons through the use of intercalary months broke down during the late Republic. The Roman Pontifex Maximus, the high priest, determined when an intercalary month was to occur, which was supposed to happened every other year. However, the position of Pontifex Maximus was held by a member of the Roman political elite. Because the term of office of Roman magistrates was delineated by the Roman calendar year, a Pontifex Maximus could lengthen a year in which political allies were in power or shorten a year in which political opponents held office.
Julius Caesar, as Pontifex Maximus, reformed the calendar in 46 BCE. The “Julian Calendar” had a regular year of 365 days divided into the traditional twelve months. An extra day was added to Februarius every four years to keep the calendar synchronized with the seasons.
In order to realign the calendar with the seasons, 46 BCE was 445 days long, compensating for the intercalary months which had been missed during times of political strife and the chaos of the civil wars. Not surprisingly, this was also a year when Caesar held the consulship.
Confusing, huh?
In order to simplify things, the narrative of the Gaius Marius Chronicle ignores the seasonal confusion of the Roman calendar before the Julian reform. In The Swabian Affair, when Insubrecus worries about being on campaign against Ariovistus along the Rhine in early fall, he identifies the calendar month as September, just as expected.