An easy mnemonic for the number of days in each month
Thirty days has September,
April, June, and November,
All the rest have thirty-one,
Save February at twenty-eight,
But leap year, coming once in four,
February then has one day more.
It's cute but mostly unnecessary. It doesn't take a whole lot to remember that February has 28 days in a common year, and 29 days in a leap year, which happens every four years.
So the only valuable info in that poem is this section: September, April, June, and November which still requires some effort to recall. To make it stick, I came up with my own mnemonic back in middle school: SNAp June, and I've relied on this successfully for years.
In numeric format, these months correspond to 4 6 9/11.
All the other months consist of 31 days.