mnemonic for months with fewer than 31 days
A traditional Japanese mnemonic phrase: に (2), し (4), む (6), く (9), さむらい (士 = 十 + 一 = 11). It helps remember that February, April, June, September, and November have fewer than 31 days. The word 士 (samurai) is used because the kanji can be decomposed into 十 (10) and 一 (1), making 11.
If you remember 'nishimuku samurai,' you can quickly tell which months have fewer than 31 days.
When I was a child, I learned the number of days in each month using 'nishimuku samurai.'