Today is Mad Hatter’s Day. The Mad Hatter wore a top hat on the front of which a slip of paper with reads “10/6.” The 10/6 refers to the cost of a hat — 10 shillings and 6 pence, and later became the date and month to celebrate Mad Hatter Day. (Except since the Mad Hatter lived in England, 10/6 might refer to June 10th – but I’m not going there.)
The idiom “mad as a hatter” was around long before Carroll started writing. Colloquially used to describe an eccentric person, “mad as a hatter” is based on a problem that arose in the 1800s when hat companies used lead in the hat-making process. The lead got into their systems and they went insane, hence the term “mad as a hatter”.
The Haus of Dcaligari 