It may go back to the ancient Roman festival of Hilaria, part of which was a spring fun festival. Everybody is allowed to do very silly things on April Fool's Day, but only in the morning. April Fool's gone past, You're the biggest fool at last. When April Fool comes again You'll be the biggest fool then. Gowk is the Scottish word for cuckoo.
Cuckoos are birds that show up in the spring and are considered very silly or even stupid.
Some magazines and newspapers run fake stories on April first to trick their readers. And some families have been staging huge tricks on each other for years.