A friend recently raised a very good point. In the time it takes to memorize the poem itself as a child, one might as well memorize the months and their associated number of days. But, I digress. The following is the version I was taught. It has been permanently fixed in my memory since my early childhood with no hope of removal.

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