[The two new months of Ianuarius and Februarius] were first placed at the end of the year, but at some point came to be considered the first two months instead.
The January kalend, the start of the month of January, came to be celebrated as the new year at some point after it became the day for the inaugurating new consuls in 153 BC. Romans had long dated their years by these consulships, rather than sequentially, and making the kalends of January start the new year aligned this dating. Still, private and religious celebrations around the March new year continued for some time and there is no consensus on the question of the timing for 1 January's new status.
[The two new months of Ianuarius and Februarius] were first placed at the end of the year, but at some point came to be considered the first two months instead.
The January kalend, the start of the month of January, came to be celebrated as the new year at some point after it became the day for the inaugurating new consuls in 153 BC. Romans had long dated their years by these consulships, rather than sequentially, and making the kalends of January start the new year aligned this dating. Still, private and religious celebrations around the March new year continued for some time and there is no consensus on the question of the timing for 1 January's new status.
1: https://en.wikipedia.org/wiki/New_Year%27s_Day