You can just use the common mnemonics [1], although I reckon the leap-year form is annoyingly long and complicated - much better to rely on the shorter version,

You can just use the common mnemonics [1], although I reckon the leap-year form is annoyingly long and complicated - much better to rely on the shorter version, like we do in Italy, and think a bit more on even years. Worst case, you could have a "three-day-long" magnet to cover the last few items as necessary. [1] https://en.wikipedia.org/wiki/Thirty_Days_Hath_September https://en.wikipedia.org/wiki/Thirty_Days_Hath_September