January and February are the newest months on the calendar. They were originally inserted as the 11th and 12th months of the year on the old calendar. The old, Roman year had only 10 months with December as its last month. The root of the word, 'dec' is derived from Latin and Greek, translating as the number ten. Accordingly, the months after the harvest were simply numbered: Sept/seven, Oct/eight, Nov/nine, and, Dec/ten.
While the sun gives us our calendar year the moon gives us the months in fact the word ‘month’ is named after the moon. It is a period calculated to be one complete phase of the moon. There are roughly 12 moon phases in a calendar year.
The average calendrical month, which is one twelfth of a year, about 30.44 days, while the Moon's phase (synodic) cycle repeats on average every 29.53 days. Therefore, the timing of the Moon's phases shifts by an average of almost one day for each successive month.
The seven-day week also comes from the lunar cycle due to the four principal lunar phases namely the first quarter, full moon, last quarter, and new moon. Each of the four lunar phases is roughly 7 days or roughly 7.38 days but each varies slightly due to lunar apogee and perigee.
Many centuries of observational studies have yielded these figures but in ancient times these abilities lay in the distant future. So the Romans simply counted ten months from the start of their year at the vernal equinox but were flummoxed when deciding whether or not to add two or three months at the end.
Eventually January (Ianuarius) together with February (Februarius) were inserted on the calendar around 700 BC. March remained the beginning of the year until 153 BC when it was designated to January. This had the effect of shifting the months out of kilter with their literal meanings by two months! Everyone mistakenly believes that his was due to the insertion of July and August. The month Quintilis (5th) was renamed July in honor of Julius Caesar in 44 BC and Sextilis (6th) was renamed August in honor of Augustus in 8 BC.
So today, we have a lunar calendar superimposed on a solar calendar, but the two cycles are not synchronous. Reconciling the two was one of the greatest challenges faced by scholars and was only solved with the calendar reforms under Pope Gregory in 1582.
The 1582 calendar firmly placed January the first as the start of the New Year. However, many protestant countries were slow to adopt a “Catholic” calendar. Ireland, for example, under the control of Protestant England, held its New Year celebrations on the Feast of the Annunciation on March 25th. It continued up until 1752 when the English finally relented and started using the Gregorian calendar. It should be noted that most countries in Western Europe had officially adopted January 1st as New Year's Day even before they adopted the Gregorian calendar.
Many Orthodox churches (e.g., Russian, Coptic) still use the Julian calendar, but the Greek Orthodox church uses the Gregorian calendar and celebrates Christmas on December 25. And now breaking news –
KYIV (December 21, 2022) — Ukraine’s Orthodox worshippers have always celebrated Christmas on January 7 — but that will change for many this year, with the Orthodox Church of Ukraine (OCU) for the first time allowing its congregations to celebrate on December 25. This move creates a dividing line with Russia, which celebrates on January 7, and is likely to widen a rift between Ukraine’s two feuding churches.