On the first day, while trying to save the world, you can find a little girl (blonde with twintails). She will talk about some cat that is special, since you can see its guts. Ask more about it until she gives you a password: "Hell frozen over".
Keep playing until the clock shows 21:00, because you need to go to the zone with the warehouses. Wait there until you see the achievement pop up. Take in mind that this takes time.
If the chievo doesn´t appear, enter and exit a nearby warehouse. That increases the odds of getting it.