There was a
system from the ancient world using the segments of the
fingers to count with the thumb as a marker.
My parents used a counting-on-the-knuckles scheme for remembering the
days in each month. I don't recall the details, however.
As I've mentioned before, I use the method I learnt from one of the 'Art
of Electronics' books (possibly the lab manual). Namely to write hte
month as a 4-bit binary number (January = 0001 ... December == 1100) and
then XOR the 8's bit and the 1's bit. If the result of that XOR is 1,
then it's a 31 day month, otherwise it isn't
[30 days hath septemnber
And al the rest I can't remember]
-tony