Thirty days hath September is a traditional English mnemonic

Mnemonic

A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 rhyme, of which many variants are commonly used in English-speaking countries to remember the lengths of the month

Month

A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...

s in the Julian

Julian calendar

The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 and Gregorian

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 calendars.

Thirty days hath September,

April, June, and November;

All the rest have thirty-one,

Save February, with twenty-eight days clear,

And twenty-nine each leap year.

History

The rhyme has a long history. A medieval version is found in the 15th-century manuscript Harley 2341, in the British Library

British Library

The British Library is the national library of the United Kingdom, and is the world's largest library in terms of total number of items. The library is a major research library, holding over 150 million items from every country in the world, in virtually all known languages and in many formats,...

:

Thirtey days hath November,

Aprile, June, and September:

Of twyecescore-eightt is but eine,

And all the remnante be thrycescore-eine.

O´course Leap yare comes an´pynes,

Ev'rie foure yares, gote it ryghth.

An´twyecescore-eight is but twyecescore-nyne.

The literal translation of this medieval version would be:

Thirty days has November,

April, June, and September:

Of twenty-eight is but one,

And all the remnant is thirty-one.

Of course Leap year comes and stays,

Every four years got it right,

And twenty-eight is but twenty-nine.

Modern versions

Modern versions differ from this in that September and November are often reversed. As with any text that is still primarily transmitted orally

Oral tradition

Oral tradition and oral lore is cultural material and traditions transmitted orally from one generation to another. The messages or testimony are verbally transmitted in speech or song and may take the form, for example, of folktales, sayings, ballads, songs, or chants...

, many versions exist. The first three lines are usually similar, with many variations on the final lines relating to February. As one variation puts it:

Of twenty-eight there is but one,

And all the rest have thirty-one.:

...or others:

Thirty days hath September,

April, June, and November;

February has 28 alone,

All the rest have thirty-one;

Except there is a time,

When February has twenty-nine.

Thirty days hath September,

April, June and November.

All the rest have thirty-one,

Excepting February alone

Which has eight and one score

And every fourth year one more.

Thirty days hath September

April, June, and November.

All the rest have Thirty-One,

– well, all the rest but one.

February only has twenty-eight,

and that's just fine,

except for every fourth year,

when February has twenty-nine.

30 days hath September,

April, June and November,

All the rest have 31,

Except February alone.

Thirty days hath September,

April, June, and November;

Once short February's done,

All the rest have thirty-one.

Thirty days hath September,

April, June, and November;

All the rest have thirty-one:

Except February: it has twenty-eight we find,

unless it's leap year, then it has twenty-nine."

30 days have September,

April, June and November,

All the rest have 31,

Except for February alone

And that has 28 days clear

and 29 in each leap year

Thirty days has September,

April, June and November,

All the rest have thirty-one

Accept for February, it is done

At 28, but leap one more

When the year divides by four

Other more elaborate variations such as these commonly have musical melodies that accompany them:

Thirty days hath September,

April, June, and November,

All the rest have thirty-one,

except for February alone,

which has twenty-eight rain or shine,

but on leap year, twenty-nine.

Thirty days hath September,

April, June, and November,

All the rest have thirty-one,

except for February alone oh no,

which has three less than the others rain or shine,

but on leap years, twenty-nine.

Thirty days hath September,

April, June, and November;

All the rest have thirty-one,

except for February alone,

which has four and twenty-four,

til leap year gives it one day more.

Thirty days hath September,

April, June, and November;

All the rest have thirty-one,

except for quite contrary, February,

which has twenty-eight most of the time,

but in leap year twenty-nine.

Thirty days hath September,

April, June and November.

All the rest have thirty-one,

save for February alone.

And then in leap year that's the time

that February has twenty-nine.

Thirty days hath September,

April, June and November.

All the rest have thirty-one,

February has twenty-eight alone.

Leap year coming one in four,

February has one day more.

Another similar variation gives the final two lines as follows:

To which we twenty-eight assign,

Til leap year gives us twenty-nine.

In one Bugs Bunny Cartoon, Daffy Duck was quoted in saying:

"Thirty Days Hath September,

April, June, and Nebraska!"

Knuckles

The French typically use a mnemonic by counting on the knuckle

Knuckle

The knuckles are the joints of the fingers and toes, which are brought into prominence when the hand is clenched and a fist is made. The word is cognate to similar words in other Germanic languages, such as the Dutch "Knokkel" or German "Knöchel" , i.e., Knöchlein, the diminutive of the German...

s of one's hand to remember the numbers of days of the months.
Count knuckles as 31 days, depressions between knuckles as 30 (or 28/29) days. Start with the index finger

Index finger

The index finger, , is the first finger and the second digit of a human hand. It is located between the first and third digits, between the thumb and the middle finger...

 knuckle as January, and count one finger at a time towards the little finger

Little finger

The little finger, often called the pinky in American English, pinkie in Scottish English , or small finger in medicine, is the most ulnar and usually smallest finger of the human hand, opposite the thumb, next to the ring finger.-Muscles:There are four muscles that...

 knuckle (July), saying the months as you go. Then jump back to the index finger knuckle (now August) and continue for the remaining months.

One variant of this approach differs after reaching the little finger knuckle (July): instead of wrapping around back to the index finger, some people reverse direction and continue from the little finger knuckle (counting it for both July and August) and ending on the middle finger knuckle. Still others use two hands (as shown in the diagram, right): starting with the little finger knuckle of the left hand proceed to the left index finger knuckle, then (swapping hands) jump to the right fist's index finger knuckle for August, finishing on the knuckle of the right ring finger

Ring finger

The ring finger is the fourth digit of the human hand, and the second most ulnar finger, located between the middle finger and the little finger. It is also called digitus medicinalis, the fourth finger, digitus annularis, digitus quartus, or digitus IV in anatomy.- Etymology :According to László A...

(December).