The Sixth of October...
Mad Hatter Day is the second Silly Day of the year. Why the Sixth of October? See the Tenniel drawings of the Mad Hatter in Alice's Adventures in Wonderland - that slip of paper stuck in the Mad Hatter's hat reads In this style, 10/6-. What was that you said? Ten shillings sixpence. No way! It's a reminder that you should behave in the style of the Mad Hatter on the Sixth of October!
And in a nice bit of symmetry, Mad Hatter Day is almost but not quite exactly halfway 'round the year from April Fools' Day!
Unless you live in Mexico or Great Britain or some other country where the message would instead be read as the Tenth of June.
Twinkle, twinkle, little bat!