How many days in each month?
Seven months have 31 days, four have 30, and February has 28 — or 29 in a leap year. That's the whole answer. The harder part is remembering which is which, so tap the knuckle visualiser below and it sticks for good.
Knuckles = 31 days · dips = 30 (and February, the short dip)
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The two ways everyone remembers it
The first is the centuries-old rhyme — still the fastest way to recall the four short months:
April, June, and November.
All the rest have thirty-one,
Except February alone,
Which hath twenty-eight days clear,
And twenty-nine in each leap year.
The second is the knuckle method: make a fist and count across your knuckles and the dips between them. Every knuckle is a 31-day month, every dip a 30-day month (February being the short exception). Run January to July across one hand, then start August back on the first knuckle — which is exactly why July and August, both knuckles, are the only two 31-day months in a row.
Why are the months so uneven?
Blame Rome. The early Roman calendar, traditionally credited to Numa Pompilius around 700 BC, juggled months of 29 and 31 days (even numbers were thought unlucky) and treated February as the short, end-of-year month where adjustments were made. The lengths we use now were essentially locked in by Julius Caesar's reform of 46 BC, the Julian calendar. The leftover oddity is in the names: September, October, November and December mean the 7th, 8th, 9th and 10th months — which they were, back when the year began in March.
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The Augustus myth
You'll often read that the months once alternated neatly 31/30, until Augustus stole a day from February to pad out August — the month named after him — so it wouldn't be shorter than Julius Caesar's July. It's a tidy story, and almost certainly false. It traces to the medieval scholar Sacrobosco in the 13th century and isn't supported by the Roman record; the irregular lengths were already in place under the Julian reform, three centuries before Sacrobosco wrote. A great example of a "fact" that survives because it's satisfying, not because it's true.
February and the leap-year rule
February gets 29 days in a leap year, which keeps the calendar aligned with Earth's ~365.2422-day orbit. The rule: leap years are divisible by 4 — except century years, which must also divide by 400. So 2000 was a leap year but 1900 wasn't, and 2100 won't be. Use the year box in the visualiser to flip any February and see it for yourself.
Common questions
- timeanddate.com — month lengths in the Gregorian calendar (28/29/30/31).
- The Julian calendar reform of 46 BC under Julius Caesar — established the modern month lengths.
- Johannes de Sacrobosco, 13th century — origin of the (disputed) "Augustus rearranged the months" story.
- Gregorian leap-year rule — divisible by 4, except centuries not divisible by 400.
Twelve months, four of them short, one of them changeable — and a fist that remembers all of it for you.