Thirty days hath September,
April, June, and November;
All the rest have thirty-one,
Save February, with twenty-eight days clear,
And twenty-nine each leap year.
As well as learning the poem, you can use your knuckles for learning the number of days in each month. A raised knuckle is 31 days and the dip in between is a shorter month (Usually 30 days, but 28 (or 29 for February)).