http://tipnut.com/monthly-knuckles/
From the page: “If you don’t have a calendar on hand, here’s an easy way to figure out which months have 31 days and which have 30:
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).”