April fool

Definitions

to prank or play a joke on someone
To trick someone into believing something that is not true, often on the first day of April, known as April Fool's Day. This meaning can also be used in a broader sense to refer to any type of practical joke or playful deception.
to be easily fooled or deceived
To be gullible or naive, easily falling for others' jokes or pranks. This meaning can also be used in a more serious context to describe someone who is easily tricked or manipulated in general.
to make foolish or silly decisions
To make unwise or foolish choices, often with negative consequences. This meaning can also be used to describe someone who consistently makes poor decisions or lacks common sense.

The teacher pulled an April fool on the class by telling them they had a substitute for the rest of the semester.

This is an example of using the idiom "pulled an April fool" to mean that someone played a practical joke on others, similar to the traditional April Fool's Day pranks.
The company's announcement about a new product launch was an April fool as it turned out to be a hoax.

This is an example of using the idiom "April fool" to mean that something is a prank or a joke, especially on April Fool's Day.
The news article about a celebrity's death was an April fool, and the celebrity was actually alive and well.

This is an example of using the idiom "April fool" to mean that something is a false report or a fake news story.
My friend's claim that she could speak to ghosts was an April fool, and I knew it was just a prank.

This is an example of using the idiom "April fool" to mean that someone is lying or making a false statement.
The school's announcement about a surprise assembly turned out to be an April fool, and there was no assembly after all.

This is an example of using the idiom "April fool" to mean that something was promised but did not come true.