What the heck is it and how does it work?
For every day you do the thing you track, you flip a switch. For every month the switches are in series, so if you flip every switch in a month, the LED for the month will turn on. At first I thought I could make it smaller and therefore a bit cheaper, but that didn't turn out like I had hoped at all (see this log).
At first I thought using a red 5P (P as in position/pin) switch for week days and a 2P blue switch for weekends could be cool, but the gap between each new row made it impossible to rout that AND have numbers next to it. So I opted for 10P dip switches for most of them, one 8P dip switch and seven 1P dip switches.
I just like to use big 10mm LEDs in projects when I can, like in the #BINCL - binary coded decimal 10mm LED clock project, very haptic leds.