Calendar of Destiny is a mod for registered Wolfenstein, concocted by messrs. AryanWolf3D and Chris Chokan.
The mod sows together original Wolfenstein 3D along with a cherry-picked section of Spear of Destiny, which has been tagged on as the seventh episode.
Content has been added from Spear of Destiny, of course, including wall textures and sprites, though only one boss level is featured; the showdown with the Death Knight.
As the mod features too much original material it is still sitting on the shelf. But it is a thing, nonetheless.