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  • Writer's pictureTony Herbert

How did we get our calendar? And where would we be without Julius Caesar?

Updated: Oct 31, 2023

When was the year divided up into the months we know? And where do their names come from? And what about the weeks? And the names of the days of the week? We take all these things for granted, but the answers often take us back to the ancient Romans and specifically to the great Julius Caesar. Where would we be without him?

The basic problem we face is that the year, astronomically speaking, is 365 1/4 days long (roughly) and the month, in the sense of the time it takes the moon to orbit the Earth, 29 1/2 days long (again, unfortunately, roughly). And you can’t therefore divide the year into an exact round number of months. Hence the confusion, which our ancient ancestors struggled with.


Let’s start with months, even though the year had been divided into 12 months (which it almost is) before the Romans got in on the act. It was Julius Caesar who sorted out the confusion in 45/46 BC. Up until then, the Romans had worked with a basic year of only 355 days. They well knew that this caused problems, which were resolved in a chaotic way by a group of priests known as pontifices, who added days from time to time as they saw fit. In 63 BC Julius Caesar had, perhaps surprisingly, become chief priest, or pontifex maximus (a title much later taken over by the Popes). It was in this capacity that he regularised the system. And the system he devised is still the basis of our modern calendar. It was known as the Julian Calendar.

Under the Julian Calendar, there were 12 months, starting with January. The number of days in each month was laid down in a way like ours. They alternated between 31 and 30 days,

except that February would, according to Caesar, have 29 days and only 30 every four years, ie in a leap year.

But there was a slight hiccup. When Augustus became the first Roman Emperor (after Caesar’s death and after the defeat of Mark Antony and his friend Cleopatra), the Senate decreed that the month previously called Sextilis should be renamed Augustus. The problem was that Augustus had only 30 days, according to the strict alternating system, in contrast to the previous month July, named after Julius Caesar, which had 31 days. Shock horror! One day was taken from February and slotted into August; with the result that February has the days we know and August has the full 31. The Romans could then sleep easily in their beds.

What about the names of the months? Observant readers may have already spotted a

curiosity: August was previously called Sextilis, the Latin for sixth, but it was actually the eighth month. The reason is that the Roman year originally started with March, January and February being added later. This is also the reason why the months from September through to December are named (if you know your Latin) after the wrong number. No one has bothered to change the names - maybe sensibly, as most people don’t know Latin.

Otherwise, the months are named after Roman gods. January is named after the god Janus who, appropriately, had two faces enabling him to look forward as well as backwards.

February is, as always, an oddity - named apparently after one of those Roman festivals when everyone was allowed to get drunk and behave badly. March was named after Mars, the god of war; April after Aprilis who we don’t seem to know much about; May after Maia, a god of

farming, again encouraging your young Roman to go wild in the countryside and probably causing us (or rather, some of us) to indulge in dancing around the maypole; and June, of course after Juno, the queen of the gods. As we’ve seen, the next two were named after Julius Caesar and the Emperor Augustus respectively - but they became sort of gods. As already mentioned, the final months of the year are named after their number, albeit the wrong ones.

The Gregorian Calendar

Even Julius Caesar (and, of course, his learned advisers) didn’t get it quite right. They based their calendar on a year of exactly 365.25 days. This is wrong by 11 minutes and 14 seconds a year, which one can imagine Julius thinking was rather a minor detail. The trouble was that by the 16th century it put the calendar out by 10 days.

This was addressed in 1582 by Ugo Buoncompagno who became Pope Gregory XIII. He

modified the Julian system by very slightly reducing the number of leap years. Under the Gregorian Calendar, (I hope I’ve got this right) you don’t get a leap year in a century year, unless the first two digits are exactly divisible by four (thus the year 2000 was not a leap year; the year 2100 will be). This will apparently work for another 2,800 years, viz until 4382 AD - which should see us out. Note: I got the maths slightly wrong in the first version of this, but I hope it's right now.

There was also the immediate problem of removing the 10 days. This caused riots,

particularly in protestant countries. Protestant Germany didn’t change fully unti 1775.

We in Britain didn’t make the change until 1752, when 11 days, rather than just 10, had to be abolished, causing the inevitable riots: “Give us back our 11 days!”

Voltaire had commented about our delays:

“The English prefer their calendar to disagree with the sun than to agree with the Pope”.


Who decided that a week should have seven days? No one really knows. So far as we are concerned, it obviously comes from the book of Genesis and the story of God creating the Earth and the Heavens in seven days, including his one day of rest. But it seems that other ancient cultures had the same idea. As did the Romans, who presumably didn’t get it from the Jews. So, who knows?

The names of the days of the week come from a curious mixture of Latin and Nordic gods, mostly matching the names of the seven “planets” known to the ancients.

We start with Sunday, named obviously after the sun; and Monday, after the moon. The other days are named essentially after Roman gods, although we Anglo-Saxons have changed most of the names to the equivalent nordic god.

Tuesday is mardi in French after Mars but in Anglo-Saxon countries we change it to the nordic god of war, Tiw (no, me neither!). Wednesday (mercredi) is named after Mercury in the Latin countries, but with us it’s after Woden, the same guy we know as Wotan in Wagner’s operas; Thursday (jeudi or giovedi) after Jupiter, but changed by us to Thor, god of thunder

(Donnersstag in Germany); and Friday (vendredi or venerdi) after Venus or, for us, her nordic equivalent Freyja, also familiar to Wagner fans.

Saturday is easy for us - after Saturn, the god and the planet, presumably. But interestingly, the Italians have it as Sabato, presumably reflecting the Jewish Sabbath.

For the Jews, the Sabbath is Saturday, the last day of the week after God had done all his

creating. It was the Emperor Constantine who, having converted to Christianity in AD 312,

declared that the day after the Jewish Sabbath should be the Lord’s Day (dies domenicus) and that it should be the first, not the last, day of the week.

Tony Herbert

10 September 2023

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1 Comment

Oct 31, 2023

So interesting. Thanks Tony.

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