Most months have 31 days, as you already know. There are a few that have 30 days which you'd like to know. Probably if you get paid at the end of the month and need your money a day earlier. If you don't know the poem, here's an easy way to see what months have only 30 days using your hand.

Starting with your furthest knuckle, go through each month using the knuckle and space between the knuckles. If the month you name isn't on a knuckle, it has 30 days or less in the case of February.

You can use two hands, but may be easier with just one.

via Reddit