What does the mad hatter celebrate in his tea party?

The Mad Hatter celebrates "Unbirthday" at his tea party, which is a whimsical concept introduced in Lewis Carroll's "Alice's Adventures in Wonderland." An Unbirthday is any day that is not someone's birthday, and the Hatter, along with the March Hare and the Dormouse, revels in the idea that there are 364 Unbirthdays in a year to celebrate. This quirky celebration reflects the absurdity and nonsensical nature of Wonderland, where traditional rules and conventions are turned upside down.