Happiness is mnemonic calendar verse which actually rhymes

30 days hath September

April, June, and November.

31 the others date,

Excepting February 28.

But in a leap year we divine

February, 29.

or

Thirty days hath September,

April, June and November.

When short February's done,

All the rest have 31.

or

    Thirty days hath September,

    April, June, and November;

    Thirty-one the others date,

    Excepting February, twenty-eight;

    But in leap year we assign

    February, twenty-nine.

The earliest known example of the poem dates to approximately 1425 AD:

Thirty days hath November,

April, June, and September.

Of 28 is but one

And all the remnant 30 and 1.