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The earliest #Roman calendar is traditionally said to have been invented by King #Romulus himself. Like many ancient #calendars, it attempted to reconcile the movements of the #sun and the #moon. However, while the #Mesopotamians, #Indians, #Chinese, and #Muisca had long since worked out systems of adding extra months while the #Egyptians, #Iranians, #Mesoamericans, and #Quechua decided to disregard and moon and just use the sun, the early Romans did a little work and then threwe up their hands.

In the first #Roman #calendar, the year began on the day of the #moon's #FirstVisibleCrescent after the northward #equinox (which in #Rome is the spring equinox). Since the Romans thought #EvenNumbers were unlucky, #months could have 29 or 31 days, but there were only 10 months in the year; after Decembris, there was a long period outside any #month that the Romans just thought of as "like, #winter, I guess?"

King #NumaPompilius is said to have recognised that this was silly and introduced two new months to cover the winter period, Ianuarius and Februarius.

Now, the Romans didn't think of dates the same way we do today (or really as other people in the ancient world did, because the Romans were the worst at timekeeping). They thought of each month as having its kalends on the first day, its nones on the 5th or 7th, and its ides on the 13th or 15th, and all dates were given relative to those.

This means that the Romans had no real concept of, say, "The 28th of Martius"; they thought of that day as "three days before the kalends of Aprilis".

But there was another issue. Even with 12 months, the normal #calendar year was a little shorter than the #TropicalYear. No problem, right? Just add an extra month every 2-3 years, like basically every other culture does?

Yeah, the Romans were *the worst* at timekeeping.

The extra month in the #RomanCalendar was called #Mercedonius, and it *nominally* fell in the 2nd and 5th years of a five-year cycle.

Some say Februarius was shortened and Mercedonius lasted 27 days, others that Mercedonius was 23 days long and the rest of Februarius came after the end of Mercedonius, but neither is really the case. As far as the #Romans were concerned, the kalends of Mercedonius came 10 or 11 days after the ides of Februarius and that was that.

In principle, the position of #Mercedonius was supposed to switch back and forth in alternate #LeapYears, but this was never consistent. Even if it was, it would result in a mean year of 366.25 days, this giving a calendar that didn't track the #sun OR the #moon, because the #Romans were just *the worst* at timekeeping.

But wait! It gets worse!

See, declaring a #LeapYear was actually the decision of the #PontifexMaximus, which was an elected political position. If Pontifices Maximi would routinely declare a leap year if they liked the #consuls, and refuse to declare one if they disapproved, in order to prolong or contract the administrations of various consuls according to their own preferences and ambitions.

Plus, since a #LeapYear wasn't declared until late, outlying parts of the empire wouldn't know until months later.

This meant that by the time #JuliusCaesar became dictator, nobody in the empire could agree on what the actual date was, because the Romans SUCKED at timekeeping.

The one good thing they did was begin the #CivilDay at #midnight, so at least the length of the civil day stayed consistent throughout the year.

(Oh, and at some point they moved #NewYearsDay from the kalends of Martius to the kalends of Ianuarius, which is why New Year's Day is where it is).

Anyway, #JuliusCaesar, the 0th #emperor of #Rome, recognised that the #calendar needed to be fixed. He hired #Sosigenes, a #Greek #Egyptian, to design a new calendar.

Sosigenes basically took the #EgyptianCalendar and reworked it into something acceptable to Romans. Months now consisted of 30 or 31 days, except for Februarius, which only had 28. Every four years, an extra day would be added to Februarius. Also, the new calendar would come into effect 7 days after the #WinterSolstice.

This famously meant that the last year of the #RomanCalendar officially lasted 445 days.

If this makes it seem like the Romans were the worst at timekeeping, that it completely intentional.

The new calendar was named the #JulianCalendar after its commissioner, #JuliusCaesar, even though #Sosigenes is the one who invented it. It was one of the better algorithmic calendars at the time, and spread throughout the ancient world.

But, as we now know, the #JulianCalendar's mean year is about 11 minutes 15 seconds longer than the #TropicalYear, and by the 4th century AD, the dates of the #solstices and #equinoxes had drifted from their official dates by several days.

In the 16th century, #Pope Gregory XIII used his authority as #PontifexMaximus to reform the calendar again, because that is something #popes can do.

The #GregorianCalendar is the one used in most of the world today. It was designed by #ChristopherClavius and #AloysiusLilius, but like the #JulianCalendar, it was named after the bloke who commissioned it.

The main difference is that in the Gregorian calendar, years that are integer multiples of 400 are not leap years.

The Gregorian calendar came into force in 1568 and was promptly adopted in #Catholic countries. #Protestant and #Orthodox countries were slower, since they didn't want to use a #calendar invented by Catholics.

In 1568, 10 days were skippedin #October to recalibrate the civil calendar with the #TropicalYear. By the time #Britain gave in and adpted the Gregorian calendar in 1758, they had to sip 11 days.

While #Britain was still using the #julianCalendar, #Catholics in #Ireland had to seek permission from the #pope to celebrate festivals on the wrong days because Catholicism was, to all intents and purposes, illegal at the time.

#Sweden decided to adopt the #GregorianCalendar in 1700, but tried to skip issues with skipping days by omitting 11 #LeapYears between 1700 and 1740 until Swedish dates matched those of #Catholic countries. This completely failed, and Sweden just skipped a bnch of days in 1753. In the intervening chaos, February had 30 days in Sweden in 1704 and 1708.

#Russia adopted the Gregorian calendar in 1918 as part of #Lenin's modernisation drive. 13 days were dropped, which famously meant that the #OctoberRevolution now fell in #November.

A variant of the #julianCalendar was used in #Ottoman #Türkiye, in which the year began on 1 March. The newly-formed Republic of Türkiye officially adopted the Gregorian calendar in 1926 as part of #Atatürk's policy of modernisation and westernisation.

#SaudiArabia used the #LunarHijriCalendar for all official purposes until the 1950s. At some point, it was decided to switch to a #SolarCalendar to reduce the frequency of monthly salary payments, thus saving the government money.

Arabia adopted the Gregorian calendar, which is used throughout the world, partly because it would make #commerce easier. #OmarKhayyam had invented better calendars, but money talks and Arabians are apparently OK with Christians than the wrong type of Muslims.

There is a wild #ConspiracyTheory that Emperor Otto III and #Pope Sylvester II conspired to insert 300 fake years into history so that Otto could reign in the year 1000. This is based on the fact that the Gregorian reform moved the solstices and equinoxes to the 21st rather than the 25th of the month, but this is just because Gregory XIII wanted the solstices and equinoxes to fall on the same days they did when the #CouncilOfNicaea was convened.

In the 17th century, #IsaacNewton recognised that the #julianCalendar was deficient but the #GregorianCalendar was too #Catholic for him. He worked on a new calendar whose mean year was just a few seconds shorter than the #TropicalYear (the mean Gregorian year is about 29 seconds longer). Newton viewed this as a feature rather than a bug, because under his system, the dates of the solstices and equinoxes would gradually drift back to those when #Jesus was alive.

The #EasternOrthodox Churches never adopted the Gregorian calendar (except in #Finland). While Orthodox-majority countries use the Gregorian calendar for civil purposes, many still use the #JulianCalendar for religious purposes. You have probably heard about #UkrainianRefugees celebrating #Easter and #Christmas on different dates to those of #Catholics and #Protestants; this is because the Ukrainian Orthodox Church still uses the Julian Calendar (for now).

In 1900, Maksim Trpković revised the #JulianCalendar. Under his system, an year that yields a remainder of 0 or 400 when divided by 900 is a #LeapYear.

Milutin Milanković slightly altered this in 1923, changing the rule to years that give a remainder of 200 or 600. A mean year in this calendar is just 2 seconds off the #TropicalYear. It is used by some, but not all, #EasternOrthodox churches.

And that is the story of the modern world's New Year's Day.

@Infrapink Great Thread!

That 11 day calendar skip in 1758 I think is the one that sincerely pissed off George Washington, who under the old system was born on Feb11th but now it was said he was born on Feb22. He kept celebrating his birthday on the 11th.