Ever wonder why some months have 30 days and some 31? And what’s up with February?

It’s been a while since grade school and I have to admit I still have trouble remembering which months have 30 days and which have 31? My go-to trick involves putting my two fists together and counting my knuckles in line. Raised knuckles = 31 days. Dips between them = 30.

Who decided a year has to be 365 days, or that we need 12 months that almost perfectly alternate between 30 and 31 days (I’m looking at you July and August). And for that matter, why did February end up becoming the black sheep of the calendar?

Is it because of the Moon?

Not quite. It actually takes the moon 29.5 days to orbit the Earth, not 30 or 31. Our earliest lunar-tracking ancestors devised a calendar that followed the lunar cycle. The months began with the Kalends, or new moon. They ended with the Ides, or full moon. Each month varied between 29 and 30 days, in essence averaging to 29.5.

According to myth, Rome’s first king Romulus revised the system to ten months of 30 and 31 days. This was an attempt to base the calendar less on the moon and more on the planting season cycles. The year began with the Spring Equinox in March and ended with the Winter Equinox in December. Romans didn’t care what happened between December and March because it was the dead of winter and no one wanted to go out and do anything anyway.

Later, Rome’s second king Numa Pompulius decided to do justice to winter by adding January and February onto the year. So even though February is now our second month, it used to be the last one. Since Romans considered even numbers to be unlucky, Numa changed all the 30 day months to 29, and gave January an extra day in order to bring the year count up to a nice odd 355.

Numa’s calendar looked like this:

Martius: 31 days
Aprilius: 29 days
Maius: 31 days
Iunius: 29 days
Quintilis: 31 days
Sextilis: 29 days
September: 29 days
October: 31 days
November: 29 days
December: 29 days
Ianuarius: 29 days
Februarius: 28 days

As you can see, the only even-numbered month left was February. This made sense, because if you had to have an unlucky month, it better be the short one. February was also the month in which the Roman’s performed purification rituals to honor their dead, which I guess you could say is a bit like our Halloween. It might have made sense for that month to be cosmically “dark” in the eyes of the Romans.

What about the Sun?

The problem with a 355 day year is that it actually takes 365.2422 days for the Earth to travel around the Sun. Since the math didn’t work out between the lunar calendar and the solar (or tropical) calendar, the year would become out-of-sync with the seasons over time. This was a problem because Romans celebrated their seasonal festivals on certain dates. Harvest Fest just wasn’t the same without the actual harvest!

The first fix was to add an extra “leap month” every couple of years to reset the counts. This intercalary month was oddly tacked on after February 23 and was considered to be like a Second February. Of course, the politicians who were in charge of the calendar knew a good con when they saw it, and adjusted the system conveniently to extend their terms in office. As a result, the year was increasingly off-kilter with the seasons and no one knew what the date was.

The Julian and Gregorian Calendars

Julius Caesar decided to standardize things in 46 B.C. He declared that the calendar be entirely sun-based like the Egyptians’ was, with a 365 day year. Alternating months were given an extra day or two to bring the 355 total up to 365. As a result, we had the 31 and 30 day calendar we are used to, while February retained its adjustable 28 day count for reasons described above.

This is also where leap days were standardized to come every four years to make up the 0.2422 difference. Unfortunately, the way Julius Caesar proposed to calculate the leap years led to some error over time. By 1582, year was ten days off seasonal expectations. At that time, Pope Gregory XIII created the Gregorian Calendar system, which made leap year criteria a bit more strict:

  • The year can be evenly divided by 4;
  • If the year can be evenly divided by 100, it is NOT a leap year, unless;
  • The year is also evenly divisible by 400. Then it is a leap year.

Gregory’s band-aid fix to reset the system from this point was to simply drop the ten extra days and go straight from October 4, 1582 to October 15, 1582. Kind of a bummer for anyone whose birthday lay within those dates, but it got the seasonal calendar back on track.

Of course, because change is scary, the switch to the new Gregorian calendar took 300 years to spread to the rest of the world. As time moved on, the number of days that had to be dropped increased and countries struggled to make their math work out to meet their neighbors. Some countries were even forced to do a double leap year, thus creating the most awesome birthday ever: February 30. It wasn’t until 1927 that the last country, Turkey, made the switch to the new calendar.

Why do July and August have 31 days back-to-back?

If the Julian calendar alternated months of 31 and 30 days, why do July and August both have 31? It’s convenient after all. When the “Thirty days hath September” poem fails us, we can count the months on the backs of our fists using our knuckles for 31 days and the gaps between for 30. July and August fall right where our two index knuckles meet. Of course, that can’t be the reason.


Popular account is that the months were named after Augustus and Julius Caesar, two of Rome’s most legendary emperors. Since Augustus didn’t want to appear inferior to Julius, he declared his namesake month be given just as many days as Julius’. Unfortunately, as tidy as that story is, it has been debunked by historical record, which shows that both months had 31 days even before their names were changed to honor the emperors.

The bottom line is, in order to make the 365 calendar work out, they had an extra day to account for. So, if they had to tack an extra day on somewhere, why not add it to shortened February instead?

One theory is that February just wanted to be left alone as the screwy month that leap days could be added onto. Another is that they wisely chose not to make us suffer through another day waiting for March. With the pick of the calendar, they assigned the extra day to August, giving them one more summer festival day to enjoy.

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