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Thirty days hath September is an ancient mnemonic rhyme, of which many variants are commonly used in English-speaking countries to remember the lengths of the months in the Julian and Gregorian calendars.

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

Thirty days hath November,

April, June, and September:

Of twenty-eight is but one,

And all the remnant thirty-one.^[1]

Modern versions differ from this in two main respects. Firstly, September and November are often reversed; secondly, leap years are taken into account in an additional couplet. As with any text that is still primarily transmitted orally, many versions exist, and only the first line is now always the same. The first four lines are usually similar, being (with syllables often omitted being bracketed)

Thirty days hath September,

April, June, and [dull] November:

All the rest have thirty-one,

Except[ing] [for] February [alone],

But there is no clear preference for any of the alternative endings, such as

Which hath twenty-eight days clear,

And twenty-nine in each leap year,

or

Which has eight and a score

Until leap year gives it one day more,

or

Which hath but twenty-eight, in fine,

Till leap year make it twenty-nine.

or

Which has four and twenty-four,

And every fourth year, one day more.

Infelicitous as it may seem, it is very common to recite the first couplet followed by unrhymed, unmetrical prose:

Thirty days hath September,

April, June, and November;

All the rest have thirty-one, except February, which has twenty-eight, or twenty-nine in leap year.

Other versions also exist which differ more greatly from the modern standard. Some remain much closer to the medieval version in the third and fourth lines, as for example a version that follows "November" with

February has twenty-eight alone,

All the rest have thirty-one.

Except in leap year, that's the time

When February days have twenty-nine.

While others are more specific about the nature of leap years:

All the rest have thirty-one,

Though February, it is done

At twenty-eight, though leap one more

Whenever the year divides by four.

1. ^ Modernised text based on Luria & Hoffman, Middle English Lyrics (New York: Norton, 1974), p.109