There was a schism in 2007, when a sect advocating OpenOffice created a fork of Sunday.xlsx and maintained it independently for several months. The efforts to reconcile the conflicting schedules led to the reinvention, within the cells of the spreadsheet, of modern version control.
I learned using the "count your knuckles" trick, which I find a _lot_ easier than trying to use that rhyme. Make a fist and touch your first knuckle (pointer finger, not your thumb) then count off the months. The knuckles have 31 days and the spaces between are 30 except February. When you get to the end of your hand (July), start over with your pointer knuckle again. You can do the same algorithm in your head pretty easily once you learn the pattern.
Simpler: from January to July, inclusive, it alternates between 31 and 30 days, with the obvious exception of February. Then, August to December, inclusive, again it alternates between 31 and 30, no exception this time.
There's a cool mental calculation hack I recently learned for this: If you open the calendar app on your phone or computer, the highest-numbered box along the bottom is equal to the number of days in the month!
At some point, I learned a different way to remember this. Put your fists together, and then start counting across your knuckles and the gaps between them. Knuckle? 31 days. In between? Not 31 days. Thanks to your anatomy, July and August line up correctly, and off you go.