A date sandwich

A graph of that famous handgame to remember whether a month ends in 30 or 31 days. You enumerate the days of the months on your left fist, left-to-right and touching with your right index either a knuckle or the space between it. If the month falls on a knuckle it's a 31-days month, if it falls in a valley, it's a 30-days month. The easy to remember catch is that you repeat the last knuckle.

It was the origin of the two-colored rows of my calendars.