You’re probably reciting that old nursery rhyme in your head right now. "Thirty days hath September..." It's the only way most of us can keep it straight. Honestly, the Gregorian calendar is a bit of a disaster. We’ve all had those moments where we’re trying to schedule a meeting or check a deadline and we have to pause. Which month has 30 days again? Is it June? Wait, no, maybe July? It’s a constant struggle because our modern way of tracking time isn't actually based on clean, logical math. It’s based on thousands of years of Roman politics, ego trips by emperors, and a desperate attempt to keep the seasons from drifting into the wrong part of the year.
If you just want the quick answer, here it is. There are four. September, April, June, and November. That’s it. Just four months out of twelve. The rest—except for the chaotic outlier that is February—all have 31. It feels lopsided because it is. You've got seven months with 31 days and only four with 30. If you’re wondering why we don't just even them out, you have to look back at a time when Romans were terrified of even numbers and thought February was a month for ritual purification and bad luck.
The "Thirty Day" Club: Breaking Down the Big Four
When we look at which month has 30 days, we’re looking at a specific rhythm in the year. These months are essentially the "short" full months. They aren't as short as February, but they don't get that extra "bonus" day that July or August enjoy.
April is usually the first one that trips people up. It’s the start of spring for the Northern Hemisphere, and it’s a lean 30 days. Then you hit June, the gateway to summer. By the time you get to September, you're back to 30. Finally, November closes out the 30-day group before the year ends in a 31-day December marathon.
Why these four? Why not May or October?
There isn't a "scientific" reason like the earth's rotation for why June has 30 and July has 31. It’s mostly historical baggage. Back in the day, the Roman calendar was only 10 months long. They basically ignored winter because you couldn't farm during it, so it didn't "count." When they finally added January and February to fill the gap, they had to steal days from other months to make the math work.
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The Nursery Rhyme That Everyone Uses (Because We Have To)
Most people rely on the rhyme. You know the one. It’s been around since at least the 15th century, with versions found in old English manuscripts.
Thirty days hath September,
April, June, and November;
All the rest have thirty-one,
Excepting February alone...
It’s effective. It works. But it’s also a testament to how unintuitive our system is. If a system requires a poem for the average person to use it, the system is probably broken. Yet, we stick with it. We’ve been using the Gregorian calendar since 1582 when Pope Gregory XIII tweaked the older Julian version because the dates were drifting away from the actual solar equinox. He needed Easter to stay in the right spot, so he chopped off some days and refined the leap year rules. But he kept the 30-day months exactly where they were.
The February Problem and the 30-Day Myth
Every once in a long while, people ask if February ever had 30 days. The answer is sort of, but not really in the way you’d think. In the Swedish calendar of 1712, they actually had a February 30. They were trying to switch from the Julian calendar to the Gregorian calendar and got their leap years totally mixed up. To fix the mistake, they added an extra leap day to their leap year. It’s the only time in recorded history a month that normally has 28 or 29 days officially hit 30.
But in our standard world? February is the reason the 30-day months feel so rare. If February had 30 days, the calendar would feel much more balanced. Instead, we have this weird "short" month that everyone forgets to account for in their billing cycles or rent payments.
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Why Don't We Just Change It?
You might think, "Hey, why don't we just make every month 30 or 31 days and have a couple of 'intercalary' days at the end of the year that don't belong to any month?" People have tried.
The International Fixed Calendar is a real thing. It was championed by Moses Cotsworth and even supported by George Eastman (the Kodak guy). In that version, every year has 13 months. Every month has exactly 28 days. Every date falls on the same day of the week every single year. The 13th month is called "Sol" and sits between June and July. It sounds perfect, right? No more wondering which month has 30 days because they all have 28.
It failed because humans hate change. Imagine trying to tell every business in the world they have to change their accounting software. Imagine birthdays shifting or the chaos of the "extra" day (Year Day) that doesn't belong to a week. We are stuck with our 30-day months because history is "sticky." We’ve inherited a system of quirks.
Practical Hacks for Remembering the 30-Day Months
If you don't like the rhyme, there's the "Knuckle Rule." It’s a classic.
- Close your fist.
- Count the months on your knuckles and the spaces between them.
- Start with your index finger knuckle as January (31 days).
- The space between knuckles is February (short).
- The next knuckle is March (31).
- The next space is April (30).
- The next knuckle is May (31).
- The next space is June (30).
- The pinky knuckle is July (31).
- Now, jump back to the start. The index knuckle is August (31).
- The next space is September (30).
- The next knuckle is October (31).
- The next space is November (30).
- The last knuckle is December (31).
It’s a bit weird that July and August are both 31-day months back-to-back, but that's thanks to Julius Caesar and Augustus Caesar wanting their namesake months to be "long" and important. Ego literally shaped the length of your work month.
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Managing Your Life Around 30-Day Months
Knowing which month has 30 days isn't just for trivia. It actually impacts your wallet.
If you are a freelancer or a business owner, a 30-day month like November means you have one less day of productivity compared to October. If you have subscriptions that renew on the 31st, they often get bumped to the 30th or the 1st of the following month, which can mess up your automated bookkeeping.
Most people just cruise through the year without thinking about it until they realize their "30-day trial" expires a day earlier than they expected.
Real-World Impact: The 30-Day Cycle
In the world of finance, "30 days" is the gold standard. Most invoices are "Net 30." This creates a weird friction in months like March or August. If you send an invoice on July 31st, "Net 30" lands you on August 30th. But if you send one on August 31st, you’re looking at September 30th.
The inconsistency of our months creates a "day-count convention" issue in banking. Some banks use a "30/360" model where they just pretend every month has 30 days regardless of reality, just to keep the interest calculations simple. They literally ignore the existence of the 31st to make the math look pretty.
Actionable Next Steps for Staying Organized
Instead of just memorizing a list, change how you interact with your calendar.
- Audit your auto-pays: Check any bills or subscriptions set for the 31st. Move them to the 25th or the 1st to avoid the "missing day" jump in April, June, September, and November.
- Use the Knuckle Method: Next time someone asks for a deadline, don't look at your phone. Use your hand. It trains your brain to internalize the year's rhythm.
- Plan your "Short" Months: Recognize that in June or September, you have 3.3% less time than in the months surrounding them. It’s a tiny difference, but for high-output projects, it matters.
- The 30-Day Challenge Trick: If you're starting a 30-day fitness or habit challenge, start it on the 1st of a 30-day month. It’s incredibly satisfying to have your challenge end exactly on the last day of the month. April, June, September, and November are your best friends for this.
The calendar is a mess of Roman history and Papal decrees. We live in a world where time is measured by uneven lumps of days. But now, at least, you know exactly which four months are the ones holding the line at 30.