Valve confirmed today that the Survival Pack, the first downloadable content for zombie co-op shooter Left 4 Dead, will arrive as planned on Tuesday, April 21st.

The Survival Pack adds Versus Mode support to the two retail campaigns currently lacking it, as well as a new multiplayer mode called Survival. In Survival mode, players must fend off endless hordes of Infected for as long as they can. You can read more about the intricacies of Survival mode here.

The survival pack will hit Xbox Live at about 2 am PST (5 PM EST) tomorrow. It'll be released for the PC version sometime later in the day.