Make a fist with your left hand. Point to your raised knuckles starting with your pointer finger- this is January.
The grooves between the knuckles (what are those called anyways?) have another amount of days (Feb, April, June).
Working way back from pinky knuckle (July), start with pinky again (August). You should end in the middle with December.
See for yourself! Another mom taught my husband this trick at a work event recently. I just love it!