Moo no moo. Moo no moo. Moo no moo. The three wise monkeys come from Japan or maybe China, or maybe not. As with many of the things we just accept in the normal course of our daily lives, a great many people know the answer – it’s just that they all know different answers.
One possibility is that you have three tiny worms in your body. Every sixty days, while you sleep, the worms wriggle off to the Gods and tell tales on you. The Gods then decide how bad you’ve been on a sliding scale from instant death (you know if you deserve this) to feeling a bit off colour. There are, naturally, ways to combat the worms. You could, for instance, perform various religious obligations and hope that the priesthood will intervene on your behalf. Cheapskates will just try to stay awake all night on the sixtieth night as the worms can’t leave until you go to sleep.
In Europe, the monasteries of the same period were a little more business minded, you needed to attend to your sins every seventh day rather than every sixtieth. I don’t think any other religion pushed for higher frequency of attendance, so every seventh day must have been all the market would bear.
The Japanese names for the monkeys are “mizaru, kikazaru, iwazaru“, strictly “Don’t see. Don’t hear. Don’t speak”. The -zaru part is actually an archaic form of ‘don’t’ and also a pun on the Japanese word for monkey.
So, the whole thing is just a clever joke really.