My wife makes a fist and counts her knuckles and divots on her left hand:

pinky Jan 31

divot Feb (28/29)

ring March 31

divot April 30

middle May 31

divot June 30

pointer July 31

(now head back the other way)

pointer August 31

divot September 30

middle October 31

divot November 30

ring December 31

I don't know where she learned it, but it's genius.