Infrapink (he/his/him)Today is #NewYearsEve by the Gregorian #calendar. The story of why this is so is at times pretty nuts, and ultimately is the result of the #Romans being terrible at timekeeping.
Strap in folks, it's time for a thread.
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.
The Roman festival of #Regifugium normally fell the day after #Terminalia, 11 days after the ides of Februarius and 5 days before the kalends of Martius. But in a leap year, Mercedonius would sometimes fall the day after Regifugium; other times, it would fall the day after Terminalia, in which case Regifugium wouldn't fall until 27 days later, 5 days before the ides of Martius.
The Romans were the *worst* at timekeeping.
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.
Toby Inkster 
@Infrapink "The 0th Emperor"... I like that. Apt.