What is the rhyme about the days in each month?

It has various forms, but the first two lines are:

"Thirty days hath September,

April, June and November..."

The rest of the rhyme says that all the rest have 31 days, except for February,

which has 28 days in most years, and 29 days in leap years.

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Most older forms have the archaic rhyming of "one" and "alone" which do not rhyme in modern English, e.g.

Thirty days hath September,April, June, and November.All the rest have thirty-one,Excepting February alone,And that has twenty-eight days clear,And twenty-nine in each leap year.

Later versions rhymed "thirty-one" with "one" or with "done" e.g.All the rest have thirty-one,February with twenty-eight's done.Save in leap year, that's the time,

When February's days are twenty-nine.

(using a near rhyme in the last two lines)

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.

Thirty days hath September is a traditional English 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. There are actually several somewhat different versions of it:

This is a common version:

Thirty days has September

April, June, and November

All the rest have thirty-one

Excepting February alone

Which only has twenty-eight days clear

And twenty-nine in each leap year

Here is one version of the rhyme attributed to Mother Goose:

Thirty days hath September,

April, June, and November

All the rest have 31,

Except for February.

Another version that rhymes perfectly is from an unknown source and date:

Thirty days hath September,

April, June, and November;

Thirty-one the others date,

Except in February, twenty-eight;

But in leap year we assign

February, twenty-nine.
30 days has September, April, June and November.

All the rest have 31 except for February which has

28 days each year and 29 each leap year