Read the book: «The root cause of the New Year celebration in January»

From the history of the ancient Roman calendar

It is believed that the tradition of celebrating the New Year in January originated in Ancient Rome. In ancient Rome, it was customary to celebrate the New Year in March, with the beginning of the vernal equinox, as with other Indo-European peoples. Back then, people used a calendar that had 10 months in it. But the legendary ancient Roman king Numa Pompilius (ruled from 715 to 673/672 BC), a Sabine by birth, who ruled after another legendary founder of Rome, Romulus, around 713 BC decided to change the established order. He used the ancient Etruscan calendar and added 2 more months and dubbed them "January" and "February" in honor of the gods Janus and Phoebus. The Numa calendar focused not on the solar cycle, but on the phases of the Moon and numbered 355 days. It was Numa Pompilius who proclaimed January the first month of the year, without specifying a specific date.

Janus is a two–faced god, similar many-faced gods existed among other Indo-Europeans (Porevit, Svyatovit, Triglav, etc.), who patronizes all beginnings. Phoebus is from the Latin Februa – "feast of purification". The name of the month of February comes from the Etruscan god of the underworld, Februus, and is associated with the purification rites (februa, februare, februum), which fell on the ancient Roman fertility festival of Lupercalia (February 15 – dies februatus), falling on the full moon according to the Old Roman lunar calendar. When it was necessary to introduce insertion months when establishing the solar-lunar cycle, these latter were inserted by the Romans between February 23 and 24 (with a 4-year cycle – in the second and fourth year). Under Julius Caesar, who introduced a four-year cycle consisting of three years of 365 and one year of 366 days, February of the latter contained 29 days, and February 23 was considered the seventh day before the March calends, February 24 was the sixth previous day, and February 25 was the sixth subsequent day before the March calends. Since there were two of these sixth days before the March calendar, the year in which February contained 29 days was called annus bissextus, literally from Latin – "twice the sixth" (hence the leap year). February was considered the last month of the year, both in the Greek and the Roman calendar that imitated it, so an extra day was inserted into the last month of the year with the only difference that the Greeks included an additional day at the end of the month, whereas the Romans for the last five days of the month.