Original is here. Used by permission.
The answer, honestly, is that the Romans had no fucking idea how to run a calendar.
Like, seriously, people notice “OCTOber” and “DECEMber” and say, “hey, those mean ‘eight’ and ‘ten’, but they’re the 10th and 12th months, what’s up with that?”.
If you’ve got a little more history, you’ll know that July and August are named after Julius and Augustus Caesar, and think, “oh, they added those two months and bumped the rest of the months back.”
July and August were actually originally Quintilis and Sextilis - the fifth month and the sixth month. They were called this because the year traditionally started in March. So they had Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December.
Martius was named for Mars; Junius was named for Juno. We have no idea what Aprilis and Maius were named after. (No, really. We have some clues but beyond that it’s just guesswork.) Then they got lazy and just numbered the months.
“But wait,” you ask, “what about January and February?” Hold onto your butts, because calling the months by their numbers? Not even close to the laziest the Roman calendar got.
Between the end of December and the beginning of Martius were 50-odd intercalary days. They didn’t have months associated with them. They were just sort of there.
I swear I am not making this up.
In addition, each month had either 30 or 31 days. I was going to say “alternated between” but I looked it up and nope, the Romans decided that was too easy, so it actually went:
Look at that. Remember it. You will look back on this and say “actually, that makes sense” after what comes next.
At the beginning of the Roman Republic, the Senate decided to fix the calendar. This was for two reasons:
1) The Romans thought the Greeks kicked ass, and wanted to emulate their calendar.
2) Count those days. You will notice that they add up to 355, which means that each year is actually ten (and change) days shorter than an actual solar year - which meant that by the time of the Republic, March was somewhere in the autumn.
So the Senate decided to do some reforming. They added two brand-new months to the calendar, Januarius and Februarius. Januarius was named after Janus, because his holiday fell about a week into the new month. (Janus was the god of doorways. We’ll come back to him.) February was named after the Februa, a feast that fell in the middle of the new month and that had, in fact, long since been replaced by Lupercalia, an identical feast on the same date with a different name For Reasons.
The Senate also added an intercalary month, Mercedonius, the Month of Wages.
Yes, an intercalary month. I want to make sure that’s clear.
They also changed the lengths of the months to better fit the Greek system. The Greeks had largely lunar months, so they alternated between 29-day and 30-day months. Once again, the Romans said, “you know, we like this, but it’s too easy”.
Look, the next part is going to go into “what the hell was wrong with them?” territory, just warning you.
See what I meant about Mercedonius being an intercalary month? It’s literally in the middle of February. Like, they got ¾ of the way through February, got bored, and decided to do something else for a month and come back later.
Also, the Romans had caught on to leap years by this point, so every fourth year, Februarius had an extra day on the end, bringing its total to 29.
I want to be clear, though, that while they’d caught on to leap days, they still had not caught on to the length of the damn year. Count those days again: it’s 378. By the time of poor Gaius Julius Caesar in 46 BC, the calendar was so fucked up that he needed three intercalary months to right it again.
Bonus: as @troubleMoney mentioned in the original thread, the priesthood - who until not long before Julius controlled the release of the calendar, meaning that people paid attention to them to know when the months started - would extend or contract years to keep politicians (who were on yearly terms) they liked in power or force politicians they didn’t like out early.
(which was ordered by our friend G.Jiddy but not, as far as we know, actually created by him) did three important things.
First, it added those three intercalary months to put the year back where it was supposed to be (March had slid around to the dead of winter).
Second, it got rid of Mercedonius, putting the year back at 355 days.
Third, it scattered ten new days throughout the year, which gave us the calendar we know today.
Julius’s reforms still weren’t quite right - the length of a year is just a fraction shorter than 365.25 days, which forced the Gregorian reform of 1582 (and hey, I remembered that year right on the first try). But it was good enough for government work, as they say.
(Incidentally, the Senate voted after Gaius Julius Caesar’s death to rename Quintilis after him because he was born then, and likewise Sextilis after Augustus Caesar. The Caesars themselves had little to do with it. I mean, obviously G.Jiddy couldn’t possibly have; he was dead at the time.)
A couple reasons. First, it never did (in the Roman tradition, anyway). It originally started in March, which contained the spring equinox but didn’t start on it.
The start of the year was moved back to January for political reasons. Remember Janus, the god of doorways? It was considered auspicious for consuls to change out near his festival. His festival was nearest the kalends of January. So consuls wanted to start on the kalends of Januarius so they could start their term with an offering to the god of doorways, who would then grant an auspicious transition between consuls.
So why didn’t the kalends of Januarius get moved back to the winter solstice? Because the Christians really wanted everyone to be Christian.
Lots and lots of European civilizations had midwinter celebrations. Yule hadn’t been invented yet; it was still the Germanic tribes' winter-solstice celebrations that, as far as I know, we don’t really have a name for. The Celts had their own separate midwinter celebration (I am informed that it is now Meán Geimhridh or Grianstad an Gheimhridh, but they didn’t speak modern Irish back then), the Italian pagans1 had a holiday, et cetera… and the Romans had Saturnalia.
Saturnalia was originally on the 18th of December (or, as the Romans would have measured it, the 13th/12th/14th day before the kalends of Januarius), but it expanded, becoming a week-long event. This was partly because, well, people liked a party at the end of the calendar year (not to be confused with the end of the actual year pre-Republic) and partly because it was, consciously or not, taking over the non-Roman holidays, encouraging the Germans and the Celts and the pagans to join in and have fun with the Cives Romani.
And then there’s Mithras.
We don’t know a lot about Mithras. His was a mystery cult, which is not my description but an actual anthropogical term, and it was probably based on Iranian worship of the Zoroastrian god Mithra. Mithras’s cult started rising a little before Jesus’s did. Mithras was a god who was born from a rock, slew a bull, and had lunch with the sun. For some reason, the Roman legions really liked him. (Again, mystery cult. The first rule of mystery cult is you don’t leave any goddamn clues for the archaeologists.)
Mithras had a celebration of his birth around the winter solstice. That got folded in with Saturnalia too (gee, why does your god let you have two festivals?), and now an awful lot of people were worshiping Saturn and their own gods around the winter solstice, and all of a sudden it was the 4th century CE (i.e. about 1600-1700 years ago) and the Christians were getting pretty powerful, having converted Constantine in the previous century, and they’d been oppressing the Mithraic mystery religion for a while, and they decided, hey, we want everybody to celebrate our guy. So - despite that Jesus had been almost necessarily been born in the spring, when the sheep were moved into the fields and needed shepherds who would have heard the Archangel Gabriel’s announcement - they stuck the celebration of Jesus’s birth onto the winter-solstice holiday and, not even a little bit coincidentally, right smack on top of Mithras’s birthday too.
We have not.
The people of Rome liked that there was a festival around the winter solstice, but they had gotten used to the calendar starting in January. They liked the tradition of starting the new year during what had been Janus’s festival. And Janus’s festival started a week after
Mithras Jesus’s birthday. Moving the kalends of Januarius back to the winter solstice would have necessarily moved Saturnalia away from the winter solstice, and Janus’s festival toward it, and nobody wanted that. So Saturnalia Mithras Day Christmas stayed where it was, and Januarius stayed where it was. And that’s why the new year doesn’t start on the winter solstice.
1 I use “pagan” here as the Romans would have; “paganus” meant someone who lived outside the city and practiced a non-Roman religion.