From Team Fortress Wiki
The Mad Doktor is an item set for the Medic. It was added to the game in the Very Scary Halloween Special.
The set includes the following items:
This set includes Halloween-restricted items and can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay or grant its effect, 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.