Thirty days have September,
April, June, and November;
February has 28 alone,
All the rest have 31;
Except leap year, that's the time,
When February's days are 29.
The Knuckle Mnemonic
The French and Romanians typically use a mnemonic by 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 little finger knuckle as January, and count one finger or depression at a time towards the index finger knuckle (July), saying the months as you go. Then jump back to the little finger knuckle (now August) and continue for the remaining months.
One variant of this approach differs after reaching the index finger knuckle (July): instead of wrapping around back to the little finger, some people reverse direction and continue from the index finger knuckle (counting it for both July and August) and ending on the ring finger knuckle.
Still others use two hands: starting with the little finger knuckle of the left hand, proceed to the left index finger knuckle, then (swapping hands) jump to the right fist's index finger knuckle for August, finishing on the knuckle of the right ring finger (December).