Thus quoth wikipedia:
The first day of the consular term changed several times during Roman history. It became 1 January in 153 BC.
Question is: why? (And what is the source, since wiki is unsourced on this point?)
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It was due to the Second Celtiberian War.
In 154 BC, there was rebellion in Spain. Quintus Fulvius Nobilior was designated consul for the following year but could not assume office until the Ides of March. Given the military situation, the Senate decreed January 1 to be the start of the new civil year, which permitted Nobilior to be inducted and depart with his legions that much sooner.