The history of the Roman calendar and its permanence until today is a very difficult issue to be explained.
The history of the Roman calendar and its permanence until today is a very difficult issue to be explained. We must remember how at first the calendar, closely related to the agricultural works, was lunar, began in March and had only ten months.
So on the Ides of this month, on the fifteenth of March, the consular year began, and since the consuls were the commanding generals, the military campaigns also began at the same time.
Although the mythical tradition attributes it to king Numa, there was an important reform or change by the middle of the V century BC and two more months were added to the calendar, consisting now of 12.
When the military campaigns took place in the part of the Empire comprising the Lazio or in Italy, their beginning in mid-March made sense, but when the war was taking place far from Rome, the consuls, whose term lasted a year, barely had time to move out to the fields where the military maneuvers or operations were taking place and end the war and go home before winter.
This was evident due to the Celtiberian wars in Hispania. Therefore, the Senate agreed in the year 154 B.C. that next year, the year 153 B.C., the consular year was going to begin on the first of January. So did the consul Quintus Fulvius Nobilior, who went to Segeda, near the modern-day city of Calatayud, in the current Community of Aragon, Spain, to war with its inhabitants, the Belli, who had fortified the city breaking the treaties they had previously signed. Anyway, in this campaign Nobilior, who was an all powerful Roman consul, was beaten by the Belli, Titii and Arevaci, pre-Roman Celtic people settled in Hispania at the time.
As is well known, the History of Rome is the story of a continuous war at one point or another of their large Empire. Rarely the "pax romana", the "Roman peace" or period of time without war could be declared.
So, since the year 153 B.C., the beginning of the consular year was established or set on the first of January, the day when the Kalends of January (Kalendis Ianuariis) start.
It is interesting the unexpected influx that those from Segeda, small Celtic population placed in the center of the far Hispania, were going to have with regard to the setting of the Roman calendar, by making them change it, by forcing the Romans to change their calendar, which is now the more widespread calendar throughout the world.