Thirty Days hath September

Thirty Days hath September
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


and alternately;

30 days hath September,
April, June and November,
All the rest have 31,
Excepting February alone.
Which only has but 28 days clear
And 29 in each leap year

There is also a mnemonic counting on the knuckles of one's hand to remember the numbers of days of the months. Count knuckles as 31 days, depressions between knuckles as 30 (or 28/29) days. Start with the pointer finger's knuckle as January, and count one finger at a time towards the pinky knuckle (July), saying the months as you go. Then jump back to the pointer knuckle (now August) and continue for the remaining months.

One variant of this approach differs after reaching the pinky knuckle (July): instead of wrapping around back to the pointer finger, some people reverse direction and continue from the pinky knuckle (counting it for both July and August) and ending on the middle finger knuckle. Still others use two hands: starting with the pinky knuckle of the left hand proceed to the left pointer knuckle, then (swapping hands) jump to the right fist's index knuckle for August, finishing on the knuckle of the right ring finger (December).