“30 days hath September, April, June, and November; all the rest have 31—except for February alone.” That’s true, but the rhythm in the February part just doesn’t flow, and is there anything intuitive about the selection of September, April, June, and November? And in that order? This little mnemonic rhyme has always annoyed me. You might remember it in a slightly different variation (the ever-informative Wikipedia lists no less than 29 variations, many of which attempt to incorporate the fact of leap-years), but they all seem to have the same niggling issues with choppy metre and awkward phrasing. The most succinct and memorable version sums things up quite well I think: “30 days hath September; all the rest I can’t remember.”
The “knuckle method” has my vote: put your fists side-by-side, imagine a pointer above your far left knuckle, and then move the pointer from knuckle to space-between-knuckles to knuckle again as you list off the months. Months on a knuckle have 31 days; months between don’t; July and August both have 31 days as you move from the last knuckle on one hand directly to the first knuckle on the other. There’s nothing to remember except the order of the months. Sublime!
The real question we need to be asking here is why in the world do we have this silly calendar system in the first place?
I’m glad you asked! There is a fascinating history of calendars from the ancient world. Many calendar systems have been used over the centuries by different cultures, but to keep things short(ish) we’ll focus on the history that got us to the Gregorian Calendar we use today.
Way back when Rome was first being established they had a ten month calendar system that was probably based on the lunar cycle, which averages 29.5 days. Months alternated between 29 or 30 days. You will be wondering how 10 months of 29 or 30 days adds up to a year of days—they don’t. The first month was Martius (March), named after the Roman god, Mars, and it always started so that the spring equinox was in it. After Martius came Aprilis (“to open”, as in flowers and seas), Maius (after Roman goddess Maia), Iunius (probably after Roman goddess Juno), and finally Quintilis, Sextilis, September, October, November, and December. As you can see, the last months were basically just names based on their ordinal position. The last four month names in particular are still with us, and the first four are just the Roman language versions of March, April, May, and June.
So what happened after the days in December ran out? It’s a little unclear, but its seems there was a time when people just didn’t bother much about such matters until the spring, when you needed to keep better track of time for things like agriculture, warfare, and sailing. The exact starting date of the new year would often depend on politics—a leader with enough clout might want to extend his term by keeping the civil year long, or shorten an opponent’s term by cutting it short.
The Romans did eventually decide that it would be nice to keep track of the winter days too, at which point they added the months of Ianuarius and Februarius (January and February). It was also a time when even numbers were considered unlucky, so months had either 29 or 31 days, except for February, which had 28 days as was deemed fitting for the month during which the Roman festival of purification occurred. The festival was called Februa. See what they did there?
(And for the sake of completeness: Ianuarius is thought to be named after Janus, the Roman god of beginnings, but it may also have been named after Juno.)
The civil year then had 355 days. This was a problem because the civil year quickly got out of sync with the solar year, which is about 365.242181 days long (really). The Romans fixed this problem by adding an ‘intercalary month’ of 22 or 23 days between February and March every so often, again depending on the politics of the time.
You can imagine that this was all a bit messy. Julius Caesar would agree. In about 45BC (depending on the calendar you’re using) he decided to clean things up by adding a total of 10 days to all of the 29 day months to make a civil year of 365 days composed of months of 31 or 30 days. (The Roman aversion to even numbers had evidently subsided.) He left February at 28 days but established the rule that every fourth year it would have 29 days to make up the approximately quarter day that his calendar lost due to the earth’s refusal to travel around the sun in even intervals.
As an interesting side note: he placed the spring equinox on March 25. The winter equinox was celebrated on December 25. Sound familiar? Of course, there’s also January 25, on which we celebrate my mother’s birthday, but I digress.
Julius’ calendar system became widely used in the Western world, and it looks mostly familiar to us today. (He also renamed the first of the ‘number’ months to Julius, after himself, and his successor, Augustus Caesar, would later do the same with the next month, Sextilis—hence, July and August.)
Unfortunately, the Julian Calendar was still off by 11 minutes per year, meaning every few centuries the civil calendar lagged the solar year by a day. By the late 16th century, the Roman Catholic church was dealing with the conundrum that the date of Easter wasn’t coinciding nicely with the spring equinox any more—the Julian Calendar had drifted about 10 days out of sync with the solar calendar. In order to realign things, a calendar reform was proposed in the church (pontificated at the time by Pope Gregory XIII, after which the new calendar system was named). It involved regaining the 10 lost days by shortening the month when the new calendar system was instantiated, plus the new rule that every 100th year would not be a leap year, except for every 400th year which again would. This brought the error of the calendar down from 11 minutes per year to 27 seconds per year. Not bad.
It sounds simple enough, but it took centuries before the Gregorian Calendar was established as the default calendar system around the world. The most recent country to officially switch was Greece, in 1923. Saudi Arabia, Ethiopia, Nepal, Iran, and Afghanistan use other calendar systems still to this day.
The slow and staggered adoption of the Gregorian Calendar makes for lots of interesting confusion when it comes to determining proper historical dates. As each place switched to the new system it would drop a number of days from the calendar in order to align itself to the Gregorian Calendar. So, for example, when Alaska was purchased from Russia by the U.S.A. in October 1867, the Russian empire was still using the Julian Calendar. It was Friday the 6th. The next day, when Alaska joined the Gregorian Calendar used by America, it was Friday the 18th. The Friday was repeated because in addition to switching calendars, it also changed which side of the International Date Line it was on. What a weekend!
One more interesting note, just for fun: the Orthodox Church has never adopted the Gregorian Calendar, which is why many Orthodox Christians currently celebrate Christmas on January 7 (which is when December 25 occurs in the Julian Calendar). However, in 1924, the Revised Julian Calendar (RJC) was adopted by many (but not all) of the churches. In October 1924, the RJC went directly from September 30 to October 14, at which point it went from aligning with the Julian Calendar to the Gregorian Calendar. It will remain aligned until 2800, at which point it will be one day ahead.
The RJC has the distinction of being the most accurate calendar ever invented (2 seconds lost per year as opposed to the Gregorian’s 27). Unfortunately, it involves a rather awkward leap year calculation: every fourth year is a leap year, except for every hundredth year, which is a normal year, except for every year that when divided by 900 gives a remainder of 200 or 600—such a year is again a leap year. Now try to make a rhyme for that!