30 Days Hath September
A popular way of remembering how many days are in each month of the year is a poem that seems to have come out of England in the early 1500s.
Thirty days hath September,
April, June and November;
February has twenty eight alone
All the rest have thirty-one
Except in Leap Year, that's the time
When February's Days are twenty-nine.
In modern times there have been several variations on the ending, but they are substantially just torturous ways to try to get the leap year to conform to some kind of poetic form. It stubbornly resists the process. Most people simply remember the first two lines of this poem and rely on common sense and the well-known aberration of February to supply the rest.