Thirty days in September, April, June, and November. All the rest have 31, except for February, which has 28, but Leap Year gives it 29.

 

Mnemonic Device:

Thirty days in September,

April, June and November.

All the rest have thirty-one,

Except for February,

Which has twenty-eight,

But Leap Year gives it twenty-nine.

 

Explanation: to remember the number of days in each month.

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