What is the mad hatter celEbrating at his tea party?

The Mad Hatter doesn't really celebrate anything specific at his tea party in Alice in Wonderland. The party is more of a constant event, with the Mad Hatter and the March Hare always having tea time, regardless of the occasion.

There are a few things that could be interpreted as celebrations:

* The "unbirthday": The Mad Hatter makes a big deal about it being an "unbirthday" - a day that isn't anyone's birthday. This could be seen as a whimsical way of celebrating every day as a unique occasion.

* The nonsensical nature of the party: The Mad Hatter's tea party is filled with strange rules, nonsensical conversation, and unpredictable events. This could be viewed as a celebration of absurdity and imagination.

Ultimately, the true purpose of the Mad Hatter's tea party is unclear. It is simply a strange and whimsical event that reflects the Mad Hatter's eccentric personality and the overall dream-like atmosphere of Wonderland.