Description from Kotaku,
"Left 4 Dead was coming out on November 4. Was! Not is. Because it's had a slight change. In an interview with Gamekings, Valve's Gabe Newell has announced that the game will now be out on November 20. Doesn't have the "4" tie-in they were originally shooting for, but it is the tenth anniversary of Half-Life's release, so those hoping for some degree of sentimentality are still being catered for."