Make both hands into a fist and hold them out in front of you so that the knuckles are visible. Now start on a pinky and count the knuckles and valleys between them. Knuckles are 31 days, valleys are 30 (and February). When you switch between hands it doesn't count as a valley.
Left Pinky knucke: January, 31 days
Left Pinky/ring finger valley: February
Left Ring finger knuckle: march, 31
Left Ring/middle: April, 30
Left Middle: may, 31
Left Middle/index: June, 30
Left Index: July, 31
Right Index: August, 31
Right Index/middle: September, 30
Right middle: Oktober, 31
Right middle/ring: November, 30
Right ring finger knuckle: December, 31
It alternates between 31 and 30. The exception being that February got shortchanged and had to give a day to August, and it keeps alternating after that.
Due to leap year magic February has to give up yet another day, so it's either 28 or 29.
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).