Which Months Have 30, 31, and 28 Days — And Why? The Fascinating True History Behind the Calendar
Most of us learn at an early age that some months have 30 days, others have 31, and one unique month — February — has only 28 (or 29 during a leap year). But have you ever wondered why the months aren't all equal? Why not have 12 equal months of 30 days? Or a round, logical number that fits neatly into the 365-day year?
As it turns out, the answer is a strange mix of astronomy, politics, superstition, ego, and ancient Roman logic — if you can call it that. In this article, we’ll explore which months have which number of days, why the calendar ended up this way, and the surprisingly human history behind the system we all live by.
Quick Breakdown: Number of Days in Each Month
Let’s start with a refresher. Here’s how the modern Gregorian calendar divides its months:
Month
Number of Days
January
31
February
28 (29 in leap years)
March
31
April
30
May
31
June
30
July
31
August
31
September
30
October
31
November
30
December
31
So to summarize:
7 months have 31 days: January, March, May, July, August, October, December.
4 months have 30 days: April, June, September, November.
1 month has 28 days, and 29 in leap years: February.
Seems random? It is. Let’s uncover how we ended up with this odd arrangement.
Ancient Origins: Before the Romans
The concept of dividing the year into months is ancient, dating back to early civilizations like the Sumerians, Egyptians, and Babylonians. These early societies built calendars based on the cycles of the moon or solar patterns.
But lunar calendars, which consist of 12 lunar months (each about 29.5 days), only add up to around 354 days — about 11 days short of a full solar year. This caused months and seasons to drift, leading to confusion over time.
The Egyptians were among the first to use a solar calendar, dividing the year into 12 months of 30 days plus 5 bonus days, making 365 in total. Logical — but not quite what we use today.
The Roman Calendar: Where It All Went Sideways
The calendar we use today owes much of its form to ancient Rome. But the earliest Roman calendar was quite a mess.
The Original Roman Calendar (circa 700 BCE)
Attributed to Romulus, the mythical founder of Rome.
Had 10 months totaling 304 days.
The months ran from March to December.
The remaining 61 days of winter weren’t even assigned to any month!
Introduction of January and February
Later, under King Numa Pompilius, around 713 BCE:
January and February were added to the beginning of the calendar.
The total number of days became 355 — closer to a lunar year.
But the months were uneven:
Some had 29 days.
Some had 31.
February, seen as unlucky, was given just 28 days.
This decision was tied to superstition. Even numbers were thought to be unlucky, so most months were given odd numbers of days, except February — which was associated with rituals of purification and the Roman dead. It was purposely short and remained that way.
Julius Caesar Steps In — The Julian Calendar
By the time of Julius Caesar, the Roman calendar had fallen badly out of sync with the seasons. Months no longer aligned with seasonal events, and political manipulation of calendar days was common.
So in 46 BCE, Caesar introduced the Julian calendar:
A solar calendar of 365 days, divided into 12 months.
A leap year was added every 4 years to include an extra day in February (giving it 29 days).
Most months alternated between 30 and 31 days.
February was still the shortest — 28 or 29 days.
The Julian Month Arrangement
The Julian calendar months were originally arranged more evenly:
January: 31
February: 28 (29 leap years)
March: 31
April: 30
May: 31
June: 30
July (originally Quintilis): 31
August (originally Sextilis): 30
September: 31
October: 30
November: 31
December: 30
Seems reasonable, right? But then ego got involved.
Emperor Augustus and the Battle for Days
After Julius Caesar’s assassination, the month of Quintilis was renamed July in his honor.
Later, when Emperor Augustus came to power, the Senate decided to name Sextilis after him — thus creating August.
But there was a problem.
July had 31 days, and August had only 30. Augustus didn’t want his month to be inferior to Caesar’s.
So, in one of the most petty and enduring decisions in calendar history, a day was taken from February and added to August, making:
August: 31 days
February: reduced to 28 (and 29 in leap years)
To prevent having three 31-day months in a row, some of the other months were rearranged as well, resulting in the odd pattern we know today:
31 days: Jan, Mar, May, Jul, Aug, Oct, Dec
30 days: Apr, Jun, Sep, Nov
February left with the shortest span
Gregorian Calendar Reform (1582)
While the Julian calendar was a huge improvement, it overestimated the solar year by about 11 minutes. Over centuries, this caused the calendar to drift away from the actual equinoxes and solstices.
Enter Pope Gregory XIII, who introduced the Gregorian calendar in 1582:
Slight adjustment: Leap years now skipped in century years unless divisible by 400.
For example, 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not.
The reform corrected the calendar drift and established the version we use today.
But the monthly day structure remained the same — including the strange February sacrifice.
Fun Ways to Remember the Number of Days in Each Month
There are a couple of popular methods to remember how many days each month has.
1. The Rhyme
Thirty days hath September,
April, June, and November.
All the rest have thirty-one,
Excepting February alone,
Which has twenty-eight days clear,
And twenty-nine in each leap year.
2. The Knuckle Trick
Make fists with both hands and line up your knuckles:
Each knuckle represents a 31-day month.
Each valley (between knuckles) represents a 30-day month (or February).
Start with the knuckle of your left index finger as January.
Move across — when you reach the fourth knuckle (July), jump to the first knuckle on your right hand for August (also 31), then continue.
It works!
Final Thoughts: A Calendar Built by Humans, Not Logic
The calendar we live by isn’t perfect. In fact, it’s one of the most illogical, political, and emotionally influenced systems still in daily use. The differences in the number of days in each month are the result of ancient rituals, Roman ego, and papal reforms — not mathematical precision.
And yet, it works.
Understanding the quirky history of how we measure time gives us a deeper appreciation for the system we rely on every day — and a bit of amusement at how even the most basic things in life are shaped by human drama.
So next time you glance at your calendar and wonder why February is so short, just remember — it’s not nature’s doing. It’s Caesar and Augustus, still shaping our lives, one day at a time.