Do the MONSTERS only come out at night?

The most important rule in DYING LIGHT 2 is that the day is for humans but the night is for the infected.

That means the infected hide inside buildings and, generally, dark places during the day in order to avoid the sunrise or UV lights.

As a result, there are fewer infected during the day, but that doesn't mean you won't encounter them at all. There are still a few out there. Though weaker and slower, they still pose a threat.

In addition to the above, you can also draw the infected from their hiding places, e.g. if you make a lot of noise. They will then try to attack you... or another NPC.