Why is April 1st called April Fool’s Day? Speculations only exists, but why be called a fool?

The following Arabian Proverb may have the answer – a fool may be known by six things:
(1) anger without cause
(2) speech without profit
(3) change without progress
(4) inquiry without object
(5) putting trust in a stranger
(6) mistaking foes for friends.

Interesting list, can we relate to any? I know it did for me. And, so it is, Amen.