Many Left 4 Dead fans have been eagerly awaiting the release of its planned DLC ever since Valve first announced it. That was doubly so when it was revealed they would be making it free. There were some rumors that they might have the update finished before the end of January, and while that ended up not happening, Valve did push further sales of L4D by cutting the price down to half for a short time.
We may not have gotten the update in January, but now the company has actually posted a release deadline. If all goes according to plan, Valve will have the "Survival Pack" DLC available for download before April 21st. That date was apparently chosen as it is when the Critics Choice Edition re-release will be made available for sale, and they want to get the DLC out before that happens. Much of the content it includes has already been discussed, so now it's just a matter of waiting for it to drop.