From Team Fortress Wiki
The Can Opener is a headwear item for the Heavy. It changes the Heavy's head into one cut open along the top, revealing his team-colored brain. The "lid" is partially fastened to his head via large metal staples.
This item can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay, unless the server has Halloween mode enabled. However, it can be equipped at any time and can still be viewed on the loadout screen even when it is not visible during games.
October 27, 2011 Patch (Very Scary Halloween Special)
- The Can Opener was added to the game.