The creation of January and February and their readjustment from ending the year to beginning it happened hundreds of years before Julius Caesar’s reforms.
I’ve seen that “World Calendar” idea before except the version I saw had 30, 30, and 31 as the number of days per month in each quarter. Months would always start on Monday, Wednesday or Friday, with each quarter being identical.
“Messidor” sounds like it has a lot of notes and pictures thumbtacked to it. 
Depends; apparently Gemini (The Twins) were two separate signs once upon a time. If this is you, pick one and call him Fred. The other can be Barney.
When I become the emperor of the world, I’ll divide a year in 12 months, each with 30 days. Last 5 (or 6) days are calendae.
Each month, three 10-day weeks. Each week as follows: Monday, Tuesday, Wednesday, Midweek (day off), Fifth, Sixth, Thursday, Friday, Saturday (day off), Sunday (day off).
So, a month:
** 1 2 3 4 5 6 7 8 9 10**
11 12 13 14 15 16 17 18 19 20
21 22 23 24 25 26 27 28 29 30.
Ratio of working and non-working days is similar as in 7-day week, slightly in favour of lazing on a Sunday afternoon. (0.285714 vs. 0.3)
Furthermore, each day into 10 hours, hour into 100 minutes, minute into 100 seconds. All SI measurement units derived from a time unit changed accordingly, shoo. Friend of mine and me had fun conversations about this ages ago. He even programmed “analog” clock for Windows 3.1: Dial split into 10 hours, but 0th hour/midnight shifted 90 degrees (where on conventional dials 3 is), and it would go anti-clockwise. Because of convention that x-axis on a graph goes to the right, and trigonometric circle following sine wave values goes counterclockwise. Neat.
Churches won’t dare to complain. I’m the emperor, remember.
(Bolding mine and thanx for the earworm.)
When I become Emperor of The World, first thing is track down the guy I stopped to ask directions to Point Fermin, who replied, “Point Fermin? Where is that?”
He’s a fuckin’ dead man!
So if your birthday was on a Monday, your birthday would always be on a Monday?
I found that a bit inexplicably disturbing too. 
This doesn’t explain why China would choose to continue the seven-day week or why the Communist Block has not managed to get rid of it.
When the French Republican Calendar instituted the ten-day calendar, the strongest opposition didn’t come from the Catholic Church, which had been weakened by the revolution, but from the public who hated to see the aggregate number of free days decrease.
It may be fun to speculate, but nobody really wants to change the calendar, nobody could ever agree about how it should be changed, and the costs and confusion worldwide would be enormous.
…Every date routine in every program in every computer… every financial calculation of rent and interest payments… every monthly salary… the whole organisation of society would have to undergo a major upheaval… and how much benefit would there be? What problem would be solved?
The current calendar has worked well for 2000 years, with a small adjustment in 1582, and there’s no reason to change it. The lengths of the months are the way they are for Roman historical reasons, and the names of the months are the Roman names. A large portion of our culture comes from Rome anyway. Even half the words in the English language are derived from Latin.
To keep the Monday-Sunday cycle uninterrupted, you’ll want to insert leap-weeks, not leap-days. Every eight years there’d be an added week, independent of the 13 months, during which no monthly interest or rents would be due, no monthly wages would be paid or earned, and when there would be worldwide frolicking and gaiety. (Some stores would remain open during the leap-week, to sell condoms and other party accessories.)
We wouldn’t want the leap-weeks to add up over time and cause the movable feasts to move over-feastfully and then need to adopt something like Pope Gregory’s famous fix., To avoid that, assuming years are still labeled via the decimal system, simply omit the first leap-week of each century. This will actually cope with the 365.2422-day year more accurately than Gregory’s fix!
If the 13-month year catches on, perhaps we should switch to tridecimal arithmetic as well. This would “cut the Gordian’s Knot” and finally end the bickering between those who think our number base should be a multiple of a our finger-count, and those who prefer divisibility by 3 or 4.
In addition to the 7-day week requirement, you need to maintain months in a reasonable way.
So a leap week is going to be a problem. People being paid monthly are going to feel stiffed. Landlords want to be paid for the extra week. And on and on.
You have to maintain a small variation on the length and number of months. So no leap month either. People would go crazy if they sometimes had to pay 13 mortgage payments in one year or some such.
I see no advantage at all to having a calendar date on the same weekday. Looking up what day of the week a date falls on is so trivial compared to the disruption other changes would make.
Note that in some computer/accounting situations all that matters is the ordinal day (the number of days since the end of the previous year). I say we all switch to that: happy day 224 folks.
There’s no need to explain why China or others in the Communist Block kept the seven day week because there are other big problems with changing the calendar so simple inertia can explain it plus the desire for commerce to keep in sync with the rest of he world.
What I’m saying is were there a push to remove a 7 day week, all those who complain about “Happy Holidays” would go crazy and others who don’t object to that term would be quite upset and fight it.
Plenty of reasonable ways to reorganize months that leave 5 or 6 days left over that will be a Purge week.
Nice, but, I’m not a twin, how in the universe will I plan my next move if I don’t know which sign to follow 
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^ What’s your horoscope tell you? 
"It all started on the 13th hour, of the 13th day, of the 13th month. We were there to discuss the misprinted calendars the school had purchased. " --Marge
Love that line.
12 months is nice for seasonal symmetry. It also leads to the year being broken into fairly reasonable chunks of three months.
The real flaw though with going to a 13 month systems is largely that the system we have now isn’t particularly broken. There are minor annoyance like not knowing if Christmas falls on a Wednesday or Thursday of a given year, but overall there’s no real reason to change other than you prefer a different kind of symmetry to things. So it would be a lot of money and a lot of grief with no real benefit. There’s a reason that the French Republican Calendar only made it 12 years (and the 10 day week didn’t even make it that long) besides the fact that it was intentionally designed to disrespect people’s beliefs. It’s expensive and annoying to change something without a real reason to do so. You may say, “Look at the cleanliness of my new calendar” while I say, “Shoot, I’m going to have to change the payroll system and the quarterly tax remittances just so a calendar can look pretty.”