DOOMSDAY is the name of a new map added to TF2 today to celebrate the imminent release of the final TF2 class short, Meet the Pyro. The map commemorates the death of aeronautics monkey, Poopy Joe by having players recreate his demise. A quantity of precious Australium must be delivered to a rocket to provide enough power to launch, thus explosively initiating the space simian's ill-fated cosmonautical career.
The addition of Doomsday celebrates the launch of the three day update, which will conclude with the unveiling of Meet the Pyro on Wednesday. You can keep up with the latest updates on the Team Fortress 2 Pyromania page .