There are 4 months of the year that have exactly 30 days. They are April, June, September and November. "Thirty days hath September " and its many variants is a traditional English mnemonic rhyme to remember the lengths of the months.

One version of the rhyme is as follows:

Thirty days has September,

April, June, and November.

All the rest have thirty-one,

Except for February alone,

Which hath but twenty-eight days clear,

And twenty-nine in each leap year.

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