The Mad Hatter, a wacky character in Alice in Wonderland written by Lewis Carroll and illustrated by John Tenniel, is always portrayed wearing a quaint hat that bears the label “In This Style 10/6”. Even though this label most probably refers to the price of the hat, i.e. 10 Shillings and Six pence, many people prefer to interpret it as the date October 6 according to the way dates are written in America. Note that in the U.K. the day precedes the month in the date format and therefore Mad Hatter Day is observed on June 10.
In 1986 some computer geeks in Boulder Colorado, inspired by John Tenniel's illustrations, brought up the idea of celebrating an annual Silliness Day. Their idea, published on computer networks, was soon publicly adopted on the assumption that the harm workers do by taking a lapse to celebrate silliness is often less than the harm they do when they stick to their jobs.
The Mad Hatter day was officially adopted and widely announced on the press in 1988.
Suggestions as how to celebrate the day
Use your imagination to do something kinky that you wouldn't generally do, but make sure not to cause harm, offence or embarrassment to anybody, including your own self.