The year begins on January 1st per Julius Caesar's calendar reform of 46BC, because in Rome the year was called by the name of the elected consuls, and consuls

The year begins on January 1st per Julius Caesar's calendar reform of 46BC, because in Rome the year was called by the name of the elected consuls, and consuls were elected on January first. So basically, up until this change it was confusing for the Romans to call the year by the current consuls' names, though actually the current year ran from March to March (starting at the equinox).