From the back of the bookmark:
The Official Munchkin Bookmark for International TableTop Day! may be used in any game of Munchkin or its sequels. It may be used at any time on your turn. You may only use one bookmark (of any kind) per game unless you change sex. If you change sex, you may use a second bookmark.
1. This bookmark must be used in a store or other location displaying an International TableTop Day poster or window cling.
2. Show everyone the bookmark so they can satisfy themselves that it is what you way it is.
3. If today is International TableTop Day, you start the game with an extra Door and Treasure card in your hand. Yay, more loot!
4. You may automatically win a single combat (that is not for the winning level) by handing this bookmark back to the salesperson. If you are not in a store, give the bookmark to one of your opponents.