How to know what month has 31 days

Use your knuckles to remember each month’s days

Count the months on your knuckles and the grooves between your knuckles. Leave out your thumb knuckle. Every month that lands on a knuckle is 31 days, every month that lands on a groove between knuckles is 30 days (or 28 for February).
Clever way to remember which months have 31 days and which have 30. Here is the one hand version: 

good luck remembering