Explanation
A visual way to remember the length of each month. Put your fists together. Start on your first knuckle, go across your knuckles and the spaces in between. Say the months for each knuckle and valley. A knuckle is 31 days a valley 30; except for February, which has 28 days (29 in leap years). Also, July and August both land on a knuckle because there is no valley between your hands.
Also see: John Made Mary Jump Around Our Dog, a word-based mnemonic for just the months with 31 days.