Page "January" Paragraph 7

According to Theodor Mommsen ( The History of Rome, volume 4, The Revolution, ISBN 1-4353-4597-5, page 4 ), 1 January became the first day of the year in 600 AUC of the Roman Calendar ( 153 BC ), due to disasters in the Lusitanian War.

A Lusitanian chief called Punicus, invaded the Roman territory, defeated two Roman governors and slew their troops.

The Romans resolved to send a consul to Spain and, in order to accelerate the dispatch of aid, " they even made the new consuls enter on office two months and a half before the legal time " ( 15th of March ).