blightedpersona on 25 December 2013
It's free until 10AM PST 26/12/2013
I don't think you have to actually install/download it to keep it forever. I think all you need to do is go to the store page, click "Install game" and then it will take you to a successful purchase page and the game should now be in your library which should be enough to keep it.
But someone please correct me if that's wrong
