January 101

January is the first month of the year in the Julian and Gregorian calendars and the first of seven months to have a length of 31 days. It is, on average, the coldest month of the year within most of the Northern Hemisphere (where it is the second month of winter) and the warmest month of the year within most of the Southern Hemisphere (where it is the second month of summer). In the Southern hemisphere, January is the seasonal equivalent of July in the Northern hemisphere and vice versa.

Traditionally, the original Roman calendar consisted of 10 months totaling 304 days, winter being considered a month-less period. Around 713 BC, the semi-mythical successor of Romulus, King Numa Pompilius, is supposed to have added the months of January and February, so that the calendar covered a standard lunar year (354 days). Although March was originally the first month in the old Roman calendar, January became the first month of the calendar year either under Numa or under the   Decemvirs about 450 BC (Roman writers differ). In contrast, each specific calendar year was identified by the names of the two consuls, who entered office on May 1^{[citation needed]} or March 15 until 153 BC, from when they entered office on January 1.

According to Theodor Mommsen,^[3] 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 killed their troops. The Romans resolved to send a consul to Hispania, and in order to accelerate the dispatch of aid, "they even made the new consuls enter into office two months and a half before the legal time" (March 15).

https://en.wikipedia.org/wiki/January