The Gregorian calendar is so deeply embedded in Western culture that people take it for granted — even those of us who deal with another calendar for religious purposes. But what exactly does it stem from? Below is an attempt at a “TL;DR” summary.
All the leading calendar systems in the world can be classified in three categories: solar, lunar, and lunisolar. It was from Bernard Lewis, I believe, that I first read about the link between calendar system and type of society. Early Roman society, for instance, was agricultural so it gravitated toward a solar calendar, as the rhythm of the seasons dictated. In contrast, early Islamic society was primarily urban and nomadic, and seasons meant very little, so the Islamic calendar is lunar (to this day). Lunisolar calendars, such as the Babylonian, Jewish, Hindu, Buddhist, Chinese,… calendars, reflect mixed agricultural-urban societies.
The vagaries of the different calendar systems, then, are attempts to cope with two basic astronomical facts: the moon taking an average 29.53 days to complete one orbit around the earth (“synodic month”), and the Earth taking an average 365.24219 days to do the same around the sun (“tropical year”).
Ancient Roman calendars
The first documented calendar was attributed to Romulus, legendary co-founder (with Remus) of Rome in 753 BCE. It had 10 months of 30 or 31 days with a total of 304 days, plus a winter period. The first month was the month of vernal (spring) equinox: Martius (named after Mars, the Roman idol of war). The second through fourth months were named Aprilis (from “aperta”/open, as the Earth was ‘opened’ for seed); Maius from Maia, the idol of growth; and Junius, possibly from Juno (the wife of Jupiter). The remaining six months were prosaically named “5th” through “10th” in Latin: Quintilius, Sextilius, September, October, November, and December.
Under the 2nd of the Seven Roman Kings, Numa Pompilius, a reform took place, in which the winter period was replaced by two new months: Januarius (after the two-faced idol Janus) and 28-day Februarius (after Februa, the Roman festival of purification). As even numbers were considered unlucky, all months except Februarius had either 31 or 29 days. The total of 355 days meant that every few years, by priestly decree, an Intercalary Month had to be added to keep the calendar roughly aligned with the solar year: it was inserted between 23 February and 24 February.
Years were counted AUC (ab urbe condita, from the founding of the city, i.e., Rome).
From considering equinoxes and solstices, ancient Greek astronomers already had figured out that the solar year is something close to 365 1/4 days, even if they had it backwards as to what rotates around what.
This, combined with the difficulty of communicating a priestly decision across a far-flung empire, inspire the 46 BCE (708 AUC) calendar reform by Julius Caesar (hence “Julian”). The months acquire their current lengths, leading to a 365-day year, plus a leap years with an extra day in February every 4 years. Sounds familiar?
After the assassination of Julius Caesar, Quintilis was renamed in his honor and memory as Julius in 44 BCE; during the reign of the first “official” emperor Augustus, Sextilis was renamed in his honor 8 BCE. So now we have the year pretty much in the form we know it.
Gradually this calendar was adopted throughout the Western and Eastern roman empire, later to all lands that became Christianized.
The solstices and equinoxes shift by about 11 minutes a year in the Julian calendar. As a result, Christmas and Easter drift away from the winter solstice and the vernal equinox, respectively, by about one day every 131 years.
In 1582, Pope Gregory XIII promulgated a calendar modification in which the leap year is skipped at the turn of every century, except if the century is divisible by four. The resulting average year of 365.2425 days is tolerably close to the solar year. A onetime realignment correction was made: the days from October 5-14, 1582 were simply skipped.
Catholic countries made the switch immediately, Protestant ones followed suit later. The UK and its colonies officially switched in 1752. Where ambiguity exists about which calendar a given date refers to, dates are conventionally followed by O.S. (“Old Style”) if Julian, and N.S. (“New Style”) if Gregorian.
Countries in which the prevailing religion was Orthodox Christianity were much slower to abandon the Julian calendar. Russia only switched in 1918, February 1 (O.S.) becoming February 14 (N.S.). This is the reason, incidentally, why the October Revolution (O.S.) actually took place in November (N.S.) for the West. Greece switched even later, on March 1, 1923 (N.S.).
Of the principal churches of the Eastern Communion, the Greek Orthodox Church (including its US branch [*]) has switched to a Revised Julian Calendar proposed by M. Milanković (yes, he of the cycles!), which is effectively identical to the Gregorian until 2800. The Russian, Ukrainian, Serbian,… Orthodox churches continue to use the Julian calendar, as do the Greek Orthodox Church of Jerusalem and the Greek Old Calendarists. Until 2100, Julian dates can be converted to Gregorian ones by adding 13 days: for example, Russian Orthodox Christmas actually takes place on January 7.
In Russia since Soviet days, January 1 (N.S.) is celebrated as a secular winter holiday (“Novy God”) into which many Christmas/Yuletide traditions were co-opted.
Here’s to a Happy Gregorian New Year!
[*] Many thanks to Kia Tsakos Heavey for clarification.