Some months have 30 days while others have 31 days (except February of course) is simply because Julius Caesar designed it that way in 46 BCE. He divided 365 days into 12 months, alternating between 31 and 30 days to make the math work out.
But why did Caesar pick those lengths? Why does February get short-changed? Why do July and August back to back both have 31 days when everything else alternates? The full story starts in ancient Rome.
Reasons in a Glance
| Who | When | What Changed | Result |
| Romulus | 753 BCE |
|
|
| Numa Pompilius | 713 BCE |
|
|
| Julius Caesar | 46 BCE |
|
|
| Emperor Augustus | 8 BCE |
|
|
| Pope Gregory XIII | 1582 CE |
|
Month lengths unchanged, still Caesar's design from 46 BCE |
The Original Calendar Was Completely Different
In 753 BCE, the legendary Roman king Romulus created the very first Roman calendar and it was nothing like ours. It had only 10 months. The year started in March and ended in December. Which, by the way, is why "December" sounds like it has the word "deci" (meaning ten) in it because it used to literally be the tenth month.
That first calendar had six months of 30 days and four months of 31 days adding up to just 304 days.
King Numa Adds January & February and Unleashes Superstition
Around 713 BCE, Rome's second king, Numa Pompilius, realized that ignoring winter was impractical. So he added two new months- January and February to bring the calendar in line with the 12 cycles of the moon (a lunar year of about 355 days).
But here's where things get weird. The Romans believed that even numbers were unlucky. Bad omens. Bad energy. So Numa didn't want any month to have an even number of days.
He took a day away from each of the six 30-day months, making them 29 days (odd- safe!). Now he had 57 days left to split between the two new months. He gave January 29 days. February got 28.
Here's the catch: if you have 12 months and every single one has an odd number of days, the total is always an even number. So at least one month had to have an even number of days. Numa chose February- the month Romans dedicated to rituals for the dead. Figuring if any month was going to be "unlucky," it should be the one already associated with death, he gave it 28 days and moved on.
Julius Caesar Throws Out the Moon and Fixes Everything
By 46 BCE, the Roman calendar was an absolute mess. It had drifted so far from the actual seasons that spring festivals were happening in what was practically summer. It was embarrassing and Roman officials were even manipulating the calendar for political reasons, adding or removing days to keep their allies in office longer.
Julius Caesar decided enough was enough. He brought in a brilliant Egyptian astronomer named Sosigenes of Alexandria and asked him to design a better system. The big idea: ignore the moon entirely and base the calendar on the sun on how long it actually takes Earth to orbit the sun (365.25 days).
44 BCE: July Gets Its Name
After Caesar's death, the Roman Senate renamed the month of Quintilis ("fifth month") to July in his honor. It had 31 days which was fitting for the man who built the whole system.
8 BCE: Augustus Steals a Day from February
Caesar's heir, Emperor Augustus, had the month of Sextilis renamed August after himself. But there was a problem: August had only 30 days in Caesar's original design. July- Julius Caesar's month had 31. Augustus felt this made his month look inferior. So he simply took a day from February, bumping August to 31 days. February shrank to 28.
The Last Fix: Pope Gregory Deleted 10 Days
Caesar's system assumed a year was exactly 365.25 days. It's actually 365 days, 5 hours, 48 minutes, 45 seconds. Tiny difference but over 1,600 years it piled up to 10 full days of drift.
In 1582, Pope Gregory XIII fixed it with two moves: delete those 10 days immediately (people went to sleep on Oct 4 and woke up on Oct 15), then add a new rule- century years like 1900 skip the leap year, unless they're divisible by 400 (so 2000 kept it). Month lengths? Unchanged.
You might like to read these articles next:
