Drive Backwards?
There are many places where people still use carts and horses for transportation.
I don't happen to live in one of those places. Do you?
The meaning of the phrase - Don't put the cart before the horse - is, for those who don't get it (and those who do), that you shouldn't do things in the wrong order.
Especially, you shouldn't try to do Step B before Step A, when Step B obviously depends on Step A being done first.
To celebrate Put the Cart Before the Horse Day, you can try to do B before A and see what happens.
Pour the milk before getting a glass.
Put the eggs on the stove before setting out the frying pan.
Put the fork in your mouth before scooping up some food.
Those are just a few foody examples. I'm sure you can come up with more of the same or with non-foody examples on your own.