It might seem a rhetorical question because of its obviousness. A year has 12 months, so it is logical to place the annual restart in the first month calends of the sequence. But this was not always the case. The beginning of the year for the Romans began on March 1st, the month that paid homage to Mars, the Roman god of war… until 153 B.C.
The Belli people (part of the Celtiberian people) who lived in Segeda (Sekaisa) took advantage of a loophole in the restrictive agreements imposed after the first Celtiberian war by the Romans to build a wall around their village as there was no veto on the matter, it was only forbidden to build new fortified villages. Roman consuls saw in that work a challenge to their power, so they decided to gather an army to punish them.
But they had a problem: before starting any military campaign, the Senate had to choose the consuls, which was in March, the month of the god of war, the first month of the Roman year. Therefore, General Quintus Fulvius Nobilior, commander of the offensive against the Belli, asked the Senate to bring forward the beginning of the year to the January calends in order to reduce the wait and start the attack against the rebels earlier. So we can say that the year starts on January 1st thanks to the Celtiberians.
The alleged disobedience of the Belli served as an excuse for the Romans to start the Second Celtiberian War, in which context the famous siege of Numantia would take place.