A Microsoft press release today stated that the Left 4 Dead Survival Pack DLC will be available on April 21. This was dated for the Xbox 360 version, Valve recently stated that the PC version would be available on the same day.

The Survival Pack will be free, will add a new Survival game mode, where players are given time to dig in before fending off zombie hordes in an attempt to survive as long as possible. The DLC will include 12 new maps, 11 of them are sections of existing levels and and all new for the pack called Lighthouse