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Use your knuckles and the space between from left to right. The higher points are longer months.
The left pinky knuckle is January, the space between pinky and ring knuckle is February, the ring knuckle is March and so on. The left index knuckle will be July and you continue with August being the right hand index knuckle. All the months that land on a knuckle are 31, while everything else is 30 (except 28 or 29 for February).
There are even public schools still using corporal punishment, although the school has to have approval from the parents.