The story of the Swedish calendar
Most of us are mostly aware how the calendar works. There’s twelve months in a year, each month has 30 or 31 days, and then there’s February, which usually has 28 days and sometimes, in what is called a leap year, 29. In general, years divisible by four are leap years.
This calendar was introduced by no one else then Julius Caesar, before he became busy conquering the known world and becoming the Emperor of Rome. Before that he used to have the job title “supreme bridge builder” - the bridge connecting the human world with the world of the gods. One of the responsibilities of this role was to decide how many days to add to the end of the calendar year, because the Romans noticed that their calendar was getting misaligned with the seasons, because it was simply a bit too short. So, for every year, the supreme bridge builder had to decide how many days to add to the calendar.
Since we are talking about the Roman Republic, this was unsurprisingly misused for political gain. If the supreme bridge builder liked the people in power, he might have granted a few extra weeks. If not, no extra days. Instead of ensuring that the calendar and the seasons aligned, the calendar got even more out of whack.
Julius Caesar spearheaded a reform of the calendar, and instead of letting the supreme bridge builder decide how many days to add, the reform devised rules founded in observation and mathematical rules - leading to the calendar we still have today: twelve months each year, each with 30 or 31 days, besides February, which had 28, but every four years would have 29. This is what we today call the Julian calendar. This calendar was not perfect, but pretty good.
Over the following centuries, the role of the supreme bridge builder - or, in latin, Pontifex Maximus - transferred from the Emperor of Rome to the Bishop of Rome, the Pope. And with continuing observations over centuries it was noticed that the calendar was again getting out of sync with the seasons. So it was the Pope - Gregory XIII, later called The Great - who, in his role as Pontifex Maximus, decided that the calendar should be fixed once again. The committee he set up to work on that came up with fabulous improvements, which would guarantee to keep the calendar in sync for a much longer time frame. In addition to the rules established by the Julian calendar, every hundred years we would drop a leap year. But every four hundred years, we would skip dropping the leap year (as we did in 2000, which not many people noticed). And in 1582, this calendar - called the Gregorian calendar - was introduced.
Imagine leading a committee that comes up with rules on what the whole world would need to do once every four hundred years - and mostly having these rules implemented. How would you lead and design such a committee? I find this idea mind-blowing.
Since the time of Caesar until 1582, about fifteen centuries have passed. And in this time, the calendar was getting slightly out of sync - by one day every century, skipping every fourth. In order to deal with that shift, they decided that ten calendar days need to be skipped. Following the 4th of October 1582 was the 15th of October 1582. In 1582, there was no 5th or 14th of October, nor any of the days in between, in the countries that had the Gregorian calendar adopted.
This lead to plenty of legal discussions, mostly about monthly rents and wages: is this still a full month, or should the rent or wage be paid prorated to the number of days? Should annual rents, interests, and taxes be prorated by these ten days, or not? What day of the week should the 15th of October be?
The Gregorian calendar was a marked improvement over the Julian calendar with regards to keeping the seasons in sync with the calendar. So one might think its adoption should be a no-brainer. But there was a slight complication: politics.
Now imagine that today the Pope gets out on his balcony, and declares that, starting in five years, January to November all have 30 days, and December has 35 or 36 days. How would the world react? Would they ponder the merits of the proposal, would they laugh, would they simply adopt it? Would a country such as Italy have a different public discourse about this topic than a country such as China?
In 1582, the situation was similarly difficult. Instead of pondering the benefits of the proposal, the source of the proposal and the relation to that source became the main deciding factor. Instead of adopting the idea because it is a good idea, the idea was adopted - or not - because the Pope of the Catholic Church declared it. The Papal state, the Spanish and French Kingdoms, were first to adopt it.
Queen Elizabeth wanted to adopt it in England, but the Anglican bishops were fiercely opposed to it because it was suggested by the Pope. Other Protestant and the Orthodox countries simply ignored it for centuries. And thus there was a 5th of October 1582 in England, but not in France, and that lead to a number of confusions over the following centuries.
Ever wondered why the October Revolution started November 7? There we go. There is even a story that Napoleon won an important battle (either the Battle of Austerlitz or the Battle of Ulm) because the Russian and Austrian forces coordinated badly as the Austrians were using the Gregorian and the Russians the Julian calendar. The story is false, but it makes for a great story.
Today, the International Day of the Book is on April 23 - the death date of both Miguel de Cervantes and William Shakespeare in 1616, the two giants of literature in their respective languages - with the amusing side-effect that they actually died about two weeks apart, even though they died on the same calendar day, but in different calendars.
It wasn’t until 1923 that for most purposes all countries had deprecated the Julian calendar, and for religious purposes some still follow it - which is why the Orthodox and the Amish celebrate Christmas on January 6. Starting 2101, that should shift by another day - and I would be very curious to see whether it will, or whether by then January 6th has solidified as the Christmas date.
Possibly the most confusing story about adopting the Gregorian calendar comes from Sweden. Like most protestant countries, Sweden did not initially adopt the Gregorian calendar, and was sticking with the Julian calendar, until in 1699 they decided to switch.
Now, the idea of skipping eleven or twelve days in one go did not sound appealing - remember all the chaos that occurred in the other countries for dropping these days. So in Sweden they decided that instead of dropping the days all at once, they would drop them one by one, by skipping the leap years from 1700 until 1740, when the two calendars would finally catch up.
In 1700, February 29 was skipped in Sweden. Which didn’t bring them any closer to Gregorian countries such as Spain, because they skipped the leap year in 1700 anyway. But it brought them out of alignment with Russia - by one day.
A war with Russia started (not about the calendar, but just a week before the calendars went out of sync, incidentally), and due to the war Sweden forgot to skip the leap days in 1704 and 1708 (they had other things on their mind). And as this was embarrassing, in 1711, King Charles XII of Sweden declared to abandon the plan, and added one extra day the following year to realign it back to Russia. And because 1712 was a leap year anyway, in Sweden there was not only a February 29, but also a February 30, 1712. The only legal February 30 in history so far.
It needed not only for Charles XII to die, but also for his sister (who succeeded him) and her husband (who succeeded her) in 1751, before Sweden could move beyond that embarrassing episode, and in 1752 Sweden switched from the Julian to the Gregorian calendar, by cutting February short and ending it after February 17, following that by March 1.
Somewhere on my To-Do list, I have the wish to write a book on Wikidata. How it came to be, how it works, what it means, the complications we encountered, and the ones we missed, etc. One section in this book is planned to be about calendar models. This is an early, self-contained draft of part of that section. Feedback and corrections are very welcome.
Erdös number, update
Machine Learning and Metrology