Mad Hatter Day is observed on October 06, 2017. It was introduced in 1986 by a group of computer people in Boulder, Co and is a day to be silly and celebrate silliness. The Hatter is a fictional character in Lewis Carroll's "Alice's Adventures in Wonderland". He is often referred to as the Mad Hatter. The card or label on the Hatter's hat reads "In this style 10/6", which refers to 10 shillings and six pence, the price of the hat in pre-decimalized British money. That's why the Mad Hatter Day is celebrated on October 6.
"Mad as a hatter" is a colloquial phrase used in conversation to refer to a crazy person. In 18th and 19th century England mercury was used in the production of felt, which was used in the manufacturing of hats common of the time. People who worked in these hat factories were exposed daily to trace amounts of the metal, which accumulated within their bodies over time, causing some workers to develop dementia caused by mercury poisoning. Thus, the phrase became popular as a way to refer to someone who was perceived as insane. (With material from: Wikipedia)