April 1st, otherwise known as "April Fools Day," is a day where people normally play pranks on each other. 

Example:

A: Did you know that they're giving away a free TV today?

B: Really?

A: No, I was just kidding! April Fools!

On this day, you can fool your friends, family persons or any other known person, but it's best if the pranks are in positive way.

In the Netherlands it is also the day before which you have to fill in your income tax form of the passed year. I once had a serious question about my taxes and the office thought I was making a joke. I was furious with them. I was very short in time of making my forms complete, and they were pulling stupid jokes. I fired them as my advisors, never asked them for advice again.