Pretty self explanatory but I figured I'd make a guide since there wasn't one.
UPDATE: Night time last a very long time in this game so you don't need to hurry. You can also rest at an Outpost and change the time to night time though it's not worth it. Just keep playing and the time will change to night.
It's pretty obvious when it's night, but to make things clear. It's when the moon comes out and it gets dark... I'm kidding. You can tell by turning off your flashlight near a set of tracks. If the tracks disappear then it's night time as the only way to view animal tracks at night is with a light on.
Now find some sorry animal and kill it.