Sorry for the late reply.
You need to be at the helipad at noon on the 3rd day. (You need to get an ending so the game saves your progress.)
What you do until that time is up to you. you can go for some of the other achievements Or you can have some fun killing more zombies, you could just go to the helipad and sit there until the times comes as well. Shouldn't be to much time left by then anyway