Etymology: Five was once shown by five horizontal strokes (fingers).
This was replaced by a pictograph of a thread-reel, which served phonetically (sound) and for the fact that it replaced the work of five fingers.
Mnemonic: A REEL IS BETTER THAN FIVE FINGERS
五人 (gonin) five people
五日 (istuka) fifth day
(Based upon Henshall, 1998)