Usual Suspect Punchbowl offers this explanation as to why:
The Mad Hatter is a famous character in Lewis Caroll’s book Alice's Adventures in Wonderland.The illustration at right is from the 1951 Disney version of Alice in Wonderland. This hat clearly does say "10/6." But we suspect that this was supposed to be a price in shillings and pence, not a calendar reference.
In 1986, a group of computer technicians in Boulder, Colorado celebrated the first Mad Hatter Day. They designated the holiday as a day of silliness and year after year it has grown in popularity. Mad Hatter Day is celebrated each year on October 6 because the Mad Hatter wears a top hat labeled "In this style 10/6" in the book’s illustrations.
Someone on our crack research staff claimed to have a copy of the original Wonderland books at home and promised, further, to research the claim about the legend on the Hatter's hat. But, of course, this person failed to deliver.
Wikipedia, we can't quite make out what's written on the Mad Hatter's Hat.
Still, if you are inclined to celebrate Mad Hatter's Day, you probably don't worry about this sort of detail anyway.
Today is also National Noodle Day -- referring to the noodles in chicken noodle soup, or to pasta, or to ramen noodles, as opposed to 'noodle' as in using one's noodle.
Usual Suspect Holidays for Everyday says today is Lawyers Day, something different from Be Nice to Lawyers Day that so many of us look forward to in April, or August 31's Love Litigating Lawyers Day. Our crack research staff was unable to find a whole lot of corroboration for any observance of Lawyers Day today, although there is at least the possibility that this coming Monday might be Lovable Lawyers Day.
At The Blog of Days, we think that any lawyer who thinks that all of these 'days' are enhancing the reputation of the legal profession are probably as mad as a hatter. So there's a tie-in that way....